I've been getting to the bottom of why tracebacks aren't displaying
Cython sources on Python 3.  The problem turns out to be multi-faceted,
and fixing it right would also mean fixing some things in general, since
the way it works now is a hack.

Basically, the only reason it works at all on Python 2, is that the
`linecache` module on Python 2 has some code which, given a relative
filename (with an extension unrecognized by the import system) like
`sage/rings/integer.pyx`, it will search for this file under all
`sys.path` entries and, if found, read the source lines from that file.

This, in turn, only works for Sage because we actually install the
`.pyx` sources in the sage package.

This does not work in Python 3.  In `linecache.updatecache`, before it
tries the `sys.path` search, it checks if the module object has a
`__loader__`, and calls its `get_source()` method if it exists.  On
Python 2 this isn't a problem since modules don't necessarily have a
`__loader__`, and in particular extension modules don't.  But on the
reworked import system in Python 3, pretty much every module has a
`__loader__`--in the case of extension modules an `ExtensionFileLoader`.
But the built-in `ExtensionFileLoader` of course knows nothing about
Cython so its `get_source()` method just returns `None`.
`linecache.updatecache` assumes this is correct (why would the loader
lie?) and returns.

The simplest way to fix this is to remove the `get_source()` method from
the `ExtensionFileLoader` class. This way, Python 3 works the same way
as Python 2.

'''Upstream''':

- https://bugs.python.org/issue32797
- python/cpython#6653

URL: https://trac.sagemath.org/24681
Reported by: embray
Ticket author(s): Jeroen Demeyer, Erik Bray
Reviewer(s): Frédéric Chapoton