Fix and modernize Travis CI

[eumiro]

This should fix and modernize Travis CI:

• drop support for Python 3.4 and 3.5 (there are already f-strings in the code which are Python 3.6+)
• add support for Python 3.9 (and install libgeos-dev, see Add tox and Github Actions #131)

[eumiro]

Okay, now it works. I'd add the tox support as well, but if you are satisfied with GHA, maybe you can drop Travis altogether.