Mmf-setup

- Fix issue with `pipx` argument name changing (broken by [pipx
1159](https://github.com/pypa/pipx/pull/1159))
- Drop support for python 3.7 (EoL).

- Fix bug with warning on CoCalc.

- Add warning on CoCalc if users have not set `LC_HG_USERNAME`, or `LC_GIT_USERNAME`.

- Remove blank lines from `default.tex` so that `mmf_setup.nbinit()` works on CoCalc
(workaround for [CoCalc issue 6210][]).

[CoCalc issue 6210]: <https://github.com/sagemathinc/cocalc/issues/6210>

- Don't install mercurial etc. on CoCalc by default: these are now included in the
Default Ubuntu 20.04 image. If you need these, then explicitly specify them:

bash
mmf-setup cocalc mercurial hg-git hg-evolve jupytext black

- Recommend and support installing mmf-setup with pipx on CoCalc.
- Use new Poetry install.
- Drop support for python3.6.