Python 3.9 support

[Markus Fleschutz]

Gurobi was the reason why draf ran only on Python 3.7.

The newest version of Gurobi supports Python 3.9 (see here). So draf could theoretically run on Python 3.9

[Markus Fleschutz]

I tried to run draf on Python 3.9 with the following insights:

• ✅ matplotlib changed DivergingNorm to TwoSlopeNorm. This can just be changed.
• ⛔the latest PyPi version of gsee uses ser.clip_lower which is deprecated since pandas 0.24.0 (source) which is not compatible with Python 3.9, see here.
  • The new gsee version 0.4.0-dev is compatible with pandas 1.x but unfortunately the last commit of gsee was in July 2020.
  • So I tried to install the latest gsee version via git as documented here, but failed with
    ERROR: Could not build wheels for gsee which use PEP 517 and cannot be installed directly and Microsoft Visual C++ 14.0 or greater is required.

[Markus Fleschutz]

I added an issue on gsee for this: renewables-ninja/gsee#12