Drop support for unsupported versions of Python

[dmentipl]

As a user/developer of IMPROVER I want IMPROVER to require use of supported versions of Python so that IMPROVER is safe and secure.

The currently supported minimum version of Python is 3.9 whereas IMPROVER supports 3.6, at least according to setup.cfg. We should consider dropping support for versions of Python less than currently supported. This will require updating the conda environment files:

• conda-forge.yml
• environment_a.yml
• environment_b.yml

Related issues: conda-forge/improver-feedstock#46