mkdocstrings* microecosystem needs new pymdown-extensions source package

[Bryce Harrington]

The -proposed versions for mkdocstrings-python-handlers and mkdocstrings-python-legacy are FTBFS due to requiring newer mkdocstrings:

 Missing build dependencies: mkdocstrings (>> 0.19.0-1)

mkdocstrings in turn is FTBFS due to a requirement for a new binary dependency, python3-pymdownx, provided by the new pymdown-extensions source package, but not yet available in Debian. It is in the Debian new queue, as of June 29, 2022:

      https://ftp-master.debian.org/new.html

Presumably Debian will process this within the coming days, and this stack hopefully will resolve on its own through sync. If not, then some time prior to FF we may want to evaluate whether to pull in the new dependency, vs. remaining on the current mkdocstrings-* packages.

References:
  - https://salsa.debian.org/python-team/packages/pymdown-extensions
  - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=948324
  - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1002490