Drop support for old python versions

[mriedem]

Python 3.7 is the currently the oldest supported version:

https://endoflife.date/python

However, the setup.py, tox.ini (and maybe other locations) in this project have code for older end of life versions, e.g.:

https://github.com/python-hyper/brotlicffi/blob/main/setup.py#L100

https://github.com/python-hyper/brotlicffi/blob/main/setup.py#L112

https://github.com/python-hyper/brotlicffi/blob/main/tox.ini#L2

It'd be good to make the project current by dropping support for these old python versions, though according to https://pypi.org/project/brotlipy/#history this project hasn't been released in 5 years? Is it still being maintained?