set minimum python3 requirement

As discussed in #17 (closed):

David:

@GregAshton @ReinhardPrix should we just deprecate python 2 for this package (and set an explicit minimum version in the setup), instead of investing energy into maintaining separate package requirements?

Reinhard:

Yes, totally, I'm actually for setting the minimum to 3.7 or even the newly-released 3.8 and never looking back. Any python version can easily be installed either via pyenv or conda or similar ...

David:

3.7 could be ok, while 3.8 is a bit too bleeding edge (e.g. no lalsuite wheels are built for it yet, see https://git.ligo.org/lscsoft/lalsuite/issues/212 ); but for the moment I'd prefer even 3.5 as the minimum (to keep Debian Stretch supported).

David:

I just found out that the black style checker doesn't support 3.5 anymore, so yeah, I definitely have to admit that version is on its way out. I suggest we do one release after my current set of MRs with 3.5 as the minimum requirement, then immediately after bump to 3.6 for any further development work.