Phase out Python 3.5

[kigawas]

Since Python 3.5 reached its end of life in September 2020, I would recommend phasing it out like this:

1. In v1.1, a DeprecationWarning should be included and prompted in Python 3.5
2. From v1.2, the minimum supported version should be 3.6

[Chaostheorie]

On that note, the testing for 3.5 could also be dropped from CI to reduce workflow time. Will create a PR for this

[kigawas]

On that note, the testing for 3.5 could also be dropped from CI to reduce workflow time. Will create a PR for this

There's no hurt including 3.5 on CI, but it does need to include 3.9 since it's the current stable version.

[Chaostheorie]

3.9 is already in tests. Added it to coverage in the above-mentioned PR. My reason for removing python 3.5 from the CI is, that testing for another version costs a lot of resources due to cross-testing strategy of GINO. At the moment we not only test each version 3.5-9 for 3 Postgres versions plus some other stuff.

[Chaostheorie]

Created PR with warning for python 3.6 and lower. @kigawas is the PR to your expectations or are there adjustments required?

[kigawas]

Not exactly. You should use this:

https://docs.python.org/3/library/exceptions.html#DeprecationWarning

[Chaostheorie]

Okay, going to try to use this instead. Might try to apply it this evening.
EDIT: That was easier than I had thought. Changes were made to PR. It now uses warnings. Tests are passing so far. All good then @kigawas ?

[kigawas]

You should use sys.version_info instead and there is no need to catch exception

[Chaostheorie]

I will make the adjustments later though next time you see something just list all the problems. It's easier and faster to fix that way. Anyways, Thanks for your help, has been long since I've worked with python version warnings.

[Chaostheorie]

Changes done. Tests are running