Document Python 3.7.3 incompatibility

[rzvoncek]

Project board link

Since Medusa 0.16 we've brought in boto of a version that seems to not play ball with python 3.7.3. See this Medusa ASF slack thread for more info.

We need to write a disclaimer somewhere about this, perhaps even require pytohn 3.7.4 in setup.py (or wherever #555 needs it.)

[rzvoncek]

I've pushed a commit to #555 that handles this.
We're not documenting 3.7.3 incompatibility because we dropped 3.7 support alltogether.