Pin css-inline to v0.11

[caseyjhol]

• Support newer css-inline v0.11 (fixes Broken compability with newer versions of css_inline library #43)
• Pin to a specific minor version to prevent further issues if the css-inline API changes

[FelixSchwarz]

Thank you for your patience. I merged your changes.

I thought about adding an environment marker like css_inline >= 0.11, < 0.12; python_version >= '3.7' in setup.cfg. That way we could have kept the workflow file the same but in the end your solution works as well and gives a clear error message when a Python 3.6 user tries to install css_inline.

I'm curious why you need to add @skipIf though?

[caseyjhol]

Hrmm - I think it was maybe so somebody running Python 3.6 could still run tests locally? Maybe it's not strictly necessary.

[FelixSchwarz]

Hrmm - I think it was maybe so somebody running Python 3.6 could still run tests locally? Maybe it's not strictly necessary.

Well, with the new version requirements a Python 3.6 should not have css-inline installed so the test should have been skipped anyway. :-)

The only case I can see is a Python 3.6 user with a pre-existing environment who somehow updates his local git checkout while keeping the previous venv. But I don't think we need to support that case.

[caseyjhol]

Ah, I think at the time I was thinking we might still be able to have partial css-inline support in 3.6. I'm fine with removing skipIf here.