python 3.6 EOL

[smoia]

Relates to #410

Python 3.6, the oldest python version physiopy supports, reached its EOL just before Christmas.
On one side, dropping support would make our life easier - we could drop some tests and easily merge #410 that supports only > 3.7.
On the other, 3.6 is still the default version of some LTS distributions (e.g. Ubuntu 18.04), so removing support for 3.6 might be problematic for LTS users (we should also upgrade the OS version we use for tests).
Granted, it's not impossible to install newer versions of python on those systems - we might just want to add some documentation about it.
What to you think about it?

Outstanding questions

- Should we drop python 3.6 support?
- Do we want to write (a small piece of) documentation on how to install newer python versions on older systems?