Goold old Python 2 is dead in about 3 months as of the time of writing.

What follows is a list of roadblocks:

• chardet. We bundle version 1.0.1, from 2008. May just happen to work on py3, but I wouldn't hold my breath - done in 8e201c4.
• wxPython. Neither 2.8 nor 3.0 have Python 3 versions available. However, the work put into the wx3 upgrade was not wasted; it will make upgrading to wxPython 4.0, which does have Python 3 support, much easier - see 050391c, done in 22de7ff.
• loot-api-python. Python 3 versions are available starting from version 4.0, but see Replace libloot-python with handwritten parser #431 for the obvious problem there - done in 934f352.
• Records Refactoring #480 and Patchers Refactoring #312 are blockers, for the most part - will be much easier to proceed to Python 3 once they've progressed further
• Sub-issues:
  • Drop all remaining ur'' strings #470
  • Convert settings to VDATA3 #597
  • Replace py2exe with PyInstaller #491
• Well, the entire codebase. A decent chunk can probably be handled by automated tools, but we need to consider the output carefully - plus automated tools won't be nearly enough to catch everything. In particular:
  • Unicode strings and byte strings
    • ur'' is gone - done in 660ecbb and d43ad24
    • We should go through all our unprefixed string usages and add either b'' or u'' - done in way too many commits to list
    • StringIO needs to be changed to BytesIO where applicable - done in 7c95d05
    • str needs to be replaced with bytes or unicode depending on how it's used - done in 04835ec
    • basestring is problematic - it's gone in py3, but 2to3 will change it to str. Most usages should be replaced with unicode or bytes, but a couple really do need to handle both - done in 42b879f
    • Revisit all open uses, check if opening in bytes mode is needed - done in c120e18
  • We need to get rid of getattr calls that pass in bytes, most notably due to ModFile.__getattr__ - Patchers Refactoring #312/Records Refactoring #480, (hopefully) done in 0848872
  • Tuple parameter unpacking is gone in py3. The only place we use this is in lambdas, which is handled by 2to3 - done in 7a6b7c2
  • zip/map/filter/range need to get replaced with either the itertools/xrange variant or an equivalent comprehension - plus add list() calls if we modify the iterables in the loop - worked on in 709e146 and e8c4d43, done in a75dbc6
  • All keys and iterkeys calls can go -> done in 567f153 and 09e1633
  • items and values: Change to iteritems/itervalues, then run 2to3's dict fixer. If it changes them to iter(...), change them to viewitems/viewvalues instead - done in d22469b
  • Old-style classes need to be switched to new-style classes ahead of time to catch any issues that arise - see the commit message in fa74a1b, for example - worked on in cae844b, done in 6db5b8b
  • True division updating - for each usage, we need to consider if we want floats or integers - done in 505d26f
  • Relative imports - are we using the explicit ones everywhere already? Should mostly be caught by automated tools, but problems can arise here - done in c692662
  • Longs are gone, replaced with regular integers - a bunch of unnecessary usages in records code, which will be dropped in the course of Records Refactoring #480, any other usages should be investigated - worked on in d8d03ca, done in 197b6a2
  • ints no longer have a size limit, so sys.maxint is gone - depending on the use case, could be replaced by sys.maxsize - done in 659e5b6
  • cmp and __cmp__ are gone; usage is rare - five in ScriptParser and belt and one each in cint, balt and mods_metadata - done in 5a98eb4'

Things that are not roadblocks:

• comtypes - dropped during wxPython 3.0.2.0 upgrade
• gitpython - dropped in build script rework, see pygit2 below
• py2exe - readily supports Python 3 - we should however consider our options once we get there, see Replace py2exe with PyInstaller #491
• pyfiglet - readily supports Python 3
• pygit2 - readily supports Python 3
• pymupdf - readily supports Python 3
• pytest - readily supports Python 3
• python-lz4 - readily supports Python 3
• pywin32 - readily supports Python 3
• pyyaml - readily supports Python 3
• scandir - built into Python 3, we'll just drop this once we upgrade
• toml - readily supports Python 3

Useful tracking regexes (make sure to enable *.py mask):

• ur prefix: ur('|")[^,)]
• Old-style classes: class [^:(]+:
• Old-style super initializer calls: (?<!self\))\.__init__
• WIP regex for all old-style super calls, has tons of false positives: [A-Z]\w+\.\w+\(self(,|\))
• Absurd regex for tracking non-library imports that need updating: ^ *import (?!os|sys|re|errno|time|subprocess|io|threading|wx|wx\.adv|cPickle|argparse|atexit|codecs|ctypes|platform|shutil|traceback|codecs|chardet|lz4|win32api|win32com|yaml|tempfile|Tkinter|collections|copy|csv|copy|datetime|stat|string|struct|textwrap|scandir|inspect|pkgutil|math|gettext|locale|msgfmt|pygettext|operator|webbrowser|array|zlib|binascii|_winreg|win32gui|importlib|random|toml|pytest|glob|logging|zipfile|pygit2|py2exe)
• open usages without false positives: (?<!webbrowser\.)(?<!io\.)(?<!def )\bopen\b

Some resources:

• Supporting Python 3: An in-depth guide - and by 'in-depth' this does mean in-depth
• Porting Python 2 Code to Python 3 - a general overview from python.org
• 2to3 - automated code porting tool

Added to 308, may be addressed sooner - there's lots of factors at play here. There is also no way that I caught every issue we'll encounter on the road to py3.
Supersedes: #194 and #404
Follow-ups: #55