Camille Python 3.9 support

[IvanKolesarEquinor]

Currently we are having support up to python 3.8, but Python 3.9 is already available since Sept. 2020 (or so). We should put a support for it as well.

For Python 3.9 several small thinks would have to be thought to get a painless:

• remove Pandas version limit of 1.0.3. That will have an implication on our tests as pandas.testing.assert_series_equal now checks for series frequency as well
• need to figure out various other changes (or deprecations) from version 1.1.0 https://pandas.pydata.org/pandas-docs/stable/whatsnew/v1.1.0.html

[LauraSManns]

Merge together with #197