* make io classes inherit from typing IO classes

This makes these classes usable if type annotations are given as "IO"
or "TextIO". In the future, we'll then be able to move open() to
return a concrete class instead (python#3951).

* open: introduce concrete return types

Fixes python#3951.

We use the values of the "mode" and "buffering" arguments to figure out
the concrete type open() will return at runtime. (Compare the CPython
code in https://github.com/python/cpython/blob/master/Modules/_io/_iomodule.c#L231.)

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
to make mypy happy in Python 2, because open() is still typed as
BinaryIO unconditionally in Python 2.

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
to make mypy happy in Python 2, because open() is still typed as
BinaryIO unconditionally in Python 2.

Also made some drive-by stylistic changes.

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
because the "text mode" of open() in Python 2 doesn't really take text
(unicode).

Also made some drive-by stylistic changes.

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
because the "text mode" of open() in Python 2 doesn't really take text
(unicode).

Also made some drive-by stylistic changes.

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
because the "text mode" of open() in Python 2 doesn't really take text
(unicode).

Also made some drive-by stylistic changes.

Automatic update from web-platform-tests
Remove a hack for mypy<0.790 (#26102)

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
because the "text mode" of open() in Python 2 doesn't really take text
(unicode).

Also made some drive-by stylistic changes.
--

wpt-commits: fdc3a462396df8af5d2d8f2e5277e98817ffbb9d
wpt-pr: 26102

Automatic update from web-platform-tests
Remove a hack for mypy<0.790 (#26102)

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
because the "text mode" of open() in Python 2 doesn't really take text
(unicode).

Also made some drive-by stylistic changes.
--

wpt-commits: fdc3a462396df8af5d2d8f2e5277e98817ffbb9d
wpt-pr: 26102

Automatic update from web-platform-tests
Remove a hack for mypy<0.790 (#26102)

We have upgraded to mypy 0.790 in #26066, which brings us
python/typeshed#4146.

Unfortunately, we still have to use io.open instead of the built-in open
because the "text mode" of open() in Python 2 doesn't really take text
(unicode).

Also made some drive-by stylistic changes.
--

wpt-commits: fdc3a462396df8af5d2d8f2e5277e98817ffbb9d
wpt-pr: 26102