Bumped the version of the Material theme to 9.2.7

[alexlorenzi]

I bumped the version of the Material theme to 9.2.7 as requested in #145

Versions of Material Theme after 9.2 requires mkdocs>=1.5, but as there's no upper bound on the version range in requirements.txt the version resolved already there. I updated the lower bound in this update to ensure that the version is met.

The version pymdown-extensions needed a bump too

9.2.7 is the highest we can go without requiring a higher version of Python. Version 9.2.8 requires pymdown-extensions>=10.3 which isn't available for Python 3.7, and likewise mkdocs-material after 9.3 requires a higher python version too

[agentbellnorm]

We just have to update the chagelog and the version before merging!