This PR deprecates Python 3.6, since this version has reached EOL (see link in changelog).

Python 3.9 and 3.10 are now supported versions.
All dependencies are upgraded to versions between Python 3.7 and 3.10.

Some flake8 fixes are included.

flake8, pytest, pytest bench and cd docs && make html was successfully executed with all now supported Python versions (3.7.13, 3.8.13, 3.9.13, 3.10.4). The benchmarks where not compared!

The audiomate.corpus.io of the docs has gotten a bit bigger after upgrading Sphinx to the latest version.
The bullets of ordered lists were hidden, but visible again after also upgrading sphinx-rtd-theme to the latest version.

I didn't red the whole documentation 4 times (xD), but a quick look at all pages makes me feel like the docs looks fine.

This PR also fixes #83, though maybe a reference from the README could be added.

One Problem I observed with Python 3.7:
When installing the dev deps with pip install -e .[dev] and afterwards installing the ci deps with pip install -e .[ci] (or vice versa) an error message is shown:

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
flake8 4.0.1 requires importlib-metadata<4.3; python_version < "3.8", but you have importlib-metadata 4.12.0 which is incompatible.

In a ci environment this could be a problem when testing with pytest (Python version 3.7) and then linting with flake8.
During development this seems not to be a problem (flake8 and also pytest are running successfully despite the error).

When linting and testing (and the corresponding pip install) are executed in 2 different stages, the build should be run successfully since the error is only shown when .[ci] and .[dev] are executed consecutively.