Increase required Markdown version to 2.6.10

[lwfitzgerald]

The magiclink extension uses ancestry exclusion to prevent auto-linking
when text is already inside a link. Support for this was only added
to Markdown in version 2.6.10 (see Python-Markdown/markdown#598).

[facelessuser]

Yes, I am aware of the ancestry addition as I added it on Python Markdown and here in pymdown-extensions. But it is a backwards compatible change, meaning pymdown-extensions will work with or without it (though you will not get the benefit of the ancestry check without the latest Python Markdown).

I don't usually force specific versions unless there is good reason to force it. People sometimes have reasons they don't want to update a version. Whenever the next Python Markdown releases, I plan on forcing pymdown-extensions to use the latest Python Markdown as I will be reworking inline patterns to take advantage of the new inline methodology #215. There will be no fallback for backwards compatibility, so a force will be necessary.

[lwfitzgerald]

Although the change is backwards compatible, the test suite currently asserts the output matches that of a run with Markdown>=2.6.10 installed.

When I ran the tests in an environment with Markdown==2.6.8, the tests failed. This doesn't seem correct as the environment met the package requirements.

As the backwards compatibility is intentional, I think the test suite needs a small tweak to test against output dependent on the version of Markdown installed.

[facelessuser]

Ahh, that is a fair point. In CI, the latest is always pulled, so this hasn't been an issue, but I can see how it would be an issue in an environment that isn't clean and installing the first the time. Either I would need a separate testing requirements file, or update the project requirements...

We'll go ahead and pull this then.