Ensure rewritten modules don't inherit __future__ flags from pytest

[nicoddemus]

In a recent refactoring (#2316) I enabled all __future__ features in all pytest modules, but that has the unwanted side effect of propagating those features to compile()'d modules inside assertion rewriting, unless we pass dont_inherit=False to compile():

The optional arguments flags and dont_inherit control which future statements (see PEP 236) affect the compilation of source. If neither is present (or both are zero) the code is compiled with those future statements that are in effect in the code that is calling compile. If the flags argument is given and dont_inherit is not (or is zero) then the future statements specified by the flags argument are used in addition to those that would be used anyway. If dont_inherit is a non-zero integer then the flags argument is it – the future statements in effect around the call to compile are ignored.

Reference.

Wow it was lucky of us to catch this before the 3.1 release, I can't imagine how many suites would break by this! 😱

[The-Compiler]

Good catch! Looks like there are some other places calling compile() too though?

[coveralls]

Coverage remained the same at 92.577% when pulling 1b5f898 on nicoddemus:future-imports-rewrite into 83b241b on pytest-dev:features.

[nicoddemus]

Hmm yeah, I meant to do that after checking this fixes our suites at work and forgot to come back to review the other compile calls.

Will do it later, thanks for the reminder.

[RonnyPfannschmidt]

im fine with merging this one as is already since it improves the situation, should other compile calls need extension feel free to merge after you finished the review

thanks for the good work :)

[nicoddemus]

I did review the other usages of compile, and I think they don't need the inherit flag because they deal with only simple expressions instead of entire modules.