Closes #100 Update python version

[hugo-quantmetry]

Description

Melusine currently works only with python 3.6. Making python compatible with python 3.7 would make it possible to use DataClasses. Data Classes would be very relevant for data validation. For instance custom configurations could be validated by data classes.

Overview of the Solution: Currently, the main elements preventing Melusine from working with python 3.7 is conflicts between libraries and python versions, in particular:

Not all TensorFlow versions are compatible with python 3.8 Gensim 4+ requires numpy 1.20+ Tensorflow requires numpy ~= 1.19 Transformers 3.4 does not work with all versions of python etc

Fixes #100

Type of change

Please delete options that are not relevant.

• [ x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• [ x] Extend the test matrix to include Python 3.6 and Python 3.7

Test Configuration:

• OS: Mac OS
• Python version: 3.6, 3.7 and 3.8
• Melusine version: 2.3.1

Checklist:

• [ x] My code follows the style guidelines of this project
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas
• [ x] My changes generate no new warnings
• [ x] I have added tests that prove my fix is effective or that my feature works
• [ x] New and existing unit tests pass locally with my changes