config: typing for create_terminal_writer, re-export TerminalWriter

[blueyed]

This also imports TerminalWriter explicitly, allowing for easier
phasing out / replacing of it (via the import, which then could go
through the _pytest.compat module).

Note to self:

pick b34f07360 config: typing for create_terminal_writer
pick 443335426 Import TerminalWriter from `py.io`

[bluetech]

LGTM except the unrelated bit.

[blueyed]

I wonder if I should import it via _pytest.compat here already, so that there's really a single place where to extend/change it, and/or put it into _pytest._io in the first place - which would be just the import for now, but could later extend the class, i.e. to support "underline" markup?

[bluetech]

Importing from _pytest._io makes sense to me.

[blueyed]

Importing from _pytest._io makes sense to me.

Moved it there. Not sure if it should use __all__, explicit # noqa: F401 etc though.

[bluetech]

Mypy has a convention (a bit more than a convention actually, given the --no-implicit-reexport flag) where reexports are done like

from py.io import TerminalWriter as TerminalWriter

I don't know how flake8 would like it, but I'd do it like this with a comment explaining anyway:

# Reexport TerminalWriter from here instead of py, to make it easier to
# extend or swap our own implementation in the future.
from py.io import TerminalWriter as TerminalWriter  # noqa: F<something>

[blueyed]

Thanks, amended:

diff --git c/src/_pytest/_io/__init__.py i/src/_pytest/_io/__init__.py
index 6265353d8..047bb179a 100644
--- c/src/_pytest/_io/__init__.py
+++ i/src/_pytest/_io/__init__.py
@@ -1 +1,3 @@
-from py.io import TerminalWriter
+# Reexport TerminalWriter from here instead of py, to make it easier to
+# extend or swap our own implementation in the future.
+from py.io import TerminalWriter as TerminalWriter  # noqa: F401
diff --git c/tox.ini i/tox.ini
index fae23b882..707f239d0 100644
--- c/tox.ini
+++ i/tox.ini
@@ -189,8 +189,6 @@ markers =
 [flake8]
 max-line-length = 120
 extend-ignore = E203
-per-file-ignores =
-  src/_pytest/_io/__init__.py: F401

 [isort]
 ; This config mimics what reorder-python-imports does.