tox.ini: do not cause SyntaxWarning with py37 [ci skip]

[blueyed]

Do not cause a SyntaxError for something like:

DeprecationWarning: invalid escape sequence \w

This was happening via pdb++ when it imported pygments (and that had no
compiled .pyc file).

[RonnyPfannschmidt]

its reasonable to make those normal warnings instead of warnings as we have no controll over external packages

as a follow-up we should investigate if we can keep the error for pytest and its own tests

[blueyed]

We could use the following:

error:invalid escape sequence:DeprecationWarning:^((?![/\]pytest[/\]).)*$

But this matches through the path only (module file name), and would still throw errors from packages in .venv/… then.

A warnings filter function could be used to make use of os.path.dirname(_pytest.__file__) etc, but that would still match anything before pytest's warnings system kicks in itself anyway.

Merging as-is for now.

[asottile]

we don't have to worry for pytest itself either as pyupgrade will just fix invalid ones :)

[RonnyPfannschmidt]

@asottile good point - we still have some acceptance testing patters where we use strings in strings and might miss one that would show up in a test tho

@blueyed main reason i proposed this as a follow-up is that i believe the solution to mark those down correctly might not come easy and we migh even have to decide this one is way too tricky to be worth it

and given the point @asottile made we should naturally approach the lack of need of this check sooner or later anyway - so i propose we let it go and fix the practices around code examples in tests to support better analysis from tools over time