Custom Leaspy exceptions, PyTorch >= 1.7 compat, type hints, cleaner doc & cleaner CI

Changes

• [FEAT] Custom Leaspy exceptions raised in code: cf. leaspy.exceptions in documentation
• [COMPAT] PyTorch >=1.7 is now supported, as well as Python 3.9 (I tested torch 1.7.1, 1.8.1 & last one: 1.9.1)
• [CODE] Broad use of type annotations in Leaspy package
• [COMPAT] As a consequence support of Python 3.6 is dropped
• [CI] Cleaner CI without need for tox, with one-shot conda envs for Python 3.7, 3.8, 3.9
• [DOC] cleaned
• + Premises to clean the loss parameter (cf. !53 (closed)): to be continued (either in this MR as a new commit or anoter?)

What to start with

Many many changes due to transversal changes made (type hints + clean doc + custom exceptions). Maybe the best would be to check changes on files, commit per commit?

The code is iso-functional apart for last commit that implement preliminary work for properly handling loss setting in Leaspy.

Related issues

#40 (closed)