build: Switch to regular Python building

requirements: Drop unused grip dependency ...

Apparently I forgot to add grip support to env.sh and only noticed now.
I'm not using it so just remove it.

requirements: Unlock hypothesis ...

After a ton of refactoring newer hypothesis versions are now happy to
work without warning again. I hope.

requirements: Lock hypothesis to 6.50.1 ...

This is done because it now complains about deprecating @composite
that doesn't use draw and errors out.

tests: Use python-xdist for faster tests

Add mutmut for mutation testing ...

I haven't added a target for this yet but it's worth just adding it.

Example invocation: mutmut run --paths-to-mutate src/tokenize.py

tests: Add pytest-randomly

lint: Add flake8 command

tests: Dump pytest-timeout and clean up conftest ...

pytest-timeout just didn't integrate properly with Hypothesis, and it
was more trouble than it's worth. Dump it.

Add test harness and simple fuzz test

make: Add 'grip' target for Markdown development

make: Add 'tidy' command that runs Black code formatter