Deprecate Python 3.5 and 3.6 and support Python 3.8, 3.9, 3.10 and 3.11

[ericbn]

Currently Python 3.5, 3.6 and 3.7 are supported:

https://github.com/forcedotcom/SalesforcePy/blob/83735563b77675c2b0b761a638acd1048f760ffd/.travis.yml#L3-L7

Python 3.5 and 3.6 already reached end-of-life for longer than a year and Python 3.11 is the current stable release. See https://devguide.python.org/versions/

[caffalaughrey]

I'm leaving this open just for reference as part of migrating to actions in #67 .

[ericbn]

At this point we should also consider deprecating 3.7 and supporting 3.12. See https://devguide.python.org/versions/