Type checking improvements

[bringhurst]

• Add a PEP 561 marker file (py.typed).
• Run mypy during CI.
• Update black to avoid ImportError: cannot import name '_unicodefun' from 'click' pallets/click#2225
• Update mkdocs to avoid jinja2 3.1.0 breaks mkdocs mkdocs/mkdocs#2799
• Disable mkdocs strict mode to avoid Google Analytics v4 (gtag) mkdocs/mkdocs#2252

[bringhurst]

Note the ignore lines in mypy.ini.

#
# Please do not add any additional mypy ignores in this section!
#

[mypy-devtools.utils]
# FIXME: Remove this ignore after fixing type errors.
ignore_errors = true

[mypy-devtools.prettier]
# FIXME: Remove this ignore after fixing type errors.
ignore_errors = true

[mypy-devtools.ansi]
# FIXME: Remove this ignore after fixing type errors.
ignore_errors = true

[mypy-devtools.debug]
# FIXME: Remove this ignore after fixing type errors.
ignore_errors = true

This was done so mypy warnings/errors can be fixed in bite-sized PRs. Fixing all of those at once would be quite a large change.

If this PR is merged, a new issue should be opened to track fixing these.

[bringhurst]

Discarding due to inactivity.

[samuelcolvin]

So sorry, I missed this complete.

Feel free to reopen this and I'll review it this week.

[samuelcolvin]

Don't worry, I've fixed this in #107.

[bringhurst]

No hard feelings. 😄

I was just bulk closing old PRs.

TY for fixing it up!