Fix some warnings

[dhermes]

.../nox/nox/sessions.py:35: DeprecationWarning: invalid escape sequence \w
  path = re.sub("[^\w\s-]", "-", path).strip().lower()

.../nox/nox/sessions.py:36: DeprecationWarning: invalid escape sequence \s
  path = re.sub("[-\s]+", "-", path)

.../nox/.nox/tests-3-7/lib/python3.7/site-packages/markupsafe/__init__.py:13: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Mapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html

@theacodes I wouldn't have seen this if I didn't make some tests fail. Worth changing how pytest is invoked so that warnings turn to errors?

The third is out of our control, but has been filed / fixed already: pallets/markupsafe#99

[theacodes]

Worth changing how pytest is invoked so that warnings turn to errors?

totally.

What are we using markupsafe for?

[dhermes]

It's part of jinja2

[theacodes]

ah

…

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