Drop Python 3.7 and support Python 3.10

What does the code in the MR do ?

Python 3.7 was released back in June 2018 and will be EOL in a couple weeks, so it is time to drop support for it.

Usually we implement a deprecation cycle for this, but considering the amount of non-backward-compatible changes that are coming with the upcoming Leaspy 2.0.0, I think we can skip it...

Python 3.10 was released in October 2021 and is still not supported by Leaspy (both officially and also not tested in the CI). Let's try to see if we can make the tests run under this version.

Where should the reviewer start ?

Changes are pretty straightforward version number updates.

How can the code be tested ?

Tests should pass with added 3.10 version of Python.

When is the MR due for? (review deadline)

No due date.

What issues are linked to the MR ?

No issue.