Warn on impossible-to-match message pattern in filterwarnings - likely user error

[Zac-HD]

Suppose that you have something like the following in your Pytest config:

filterwarnings =
    ignore::The foo.bar() function is deprecated

This looks perfectly reasonable, but somehow the warning still occurs! Unfortunately - and this got me recently - the message argument is a regex pattern, and in that context the parens () are an empty group... meaning that it won't match a string which contains the literal parentheses.

I think we should use the re._parse (sre_parse on older Pythons) module to check whether filterwarnings patterns contain an empty match. Since there's no use for capture groups, it's very likely that the pattern was intended as a literal string or prefix. This is pretty easy to get wrong if you're copy-pasting the message into your warnings config! We can even suggest the re.escape()d version as a fix.

[olwenya]

I will work on this issue