fix: better error printouts

[henryiii]

This is the remainder of the changes made while working on #436. The key
change is the reworking of the error messages; if the venv builder fails,
there's much nicer error message now. Stdout/stderr are no longer hidden, which
revealed the true cause of the failure¹; a from None was added to reduce
the useless part of the printout for the builder exception (which already
"remembers" the parent exception in the constructor).

The other changes are minor cleanups noticed along the way, and are organized
by commit, feel free to look at the commits to see them individually; combined
here because our CI is large and clogs other PyPA projects.

Typing:

• chore(types): clean up over typing
• chore: bump mypy version
• chore: use sys.platform over os.name (supported by mypy)

Modernization:

• env: use subprocess.run instead of check_output
• env: update old mention of Python 2

Error printouts:

• env: better error handling for failures creating venv
• main: nicer printout of errors, including out/err
• env: from None added, reducing useless printout

Footnotes

1. This does change the type of the exception given when venv creation
   fails, but this should be a rare and broken thing anyway, so having a new
   custom exception for it seems fine for a 0.8 release. ↩

[henryiii]

This allows the function to serve as a guard below (and is correct).

[henryiii]

Moved here to make --showlocals more useful in pytest.

[layday]

Typo, should be symlinks.

[henryiii]

That hardcoded line number that might move is very annoying...

[henryiii]

(You can click "force-pushed" above to see what changed.) I've added tests for broken venv, simplified the printout code a tiny bit, removed the unneeded line pointed out by @layday, and tried to fix the moving code issue in the tests.

[henryiii]

Any reviews?