Add support for Python 3.7 and 3.8, drop 2.6, 3.3 and 3.4

[lingthio]

Let me think about this.

[and-semakin]

Of course, I've just created this issue to not lose the idea :)

[and-semakin]

We had conversation with @lingthio about supported versions on email, I just want to put here my points:

• Flask, our upstream project, currently supports only 2.7 and 3.5+;
• Flask-User, according to PyPI stats, has approximately 0 downloads in last 120 days from Python 2.6 and 3.3 and a very few downloads from 3.4;
• each old version of Python restricts us from what language features we can use; for example, if we support Python 2.6 then we can't use set and dict comprehensions and many other useful little things.

So I would like to drop support for at least Python 2.6 and 3.3 as soon as possible. Probably we can also drop support for Python 3.4 as it is also recently reached its EOL.

[lingthio]

First off: A resound "Yes" on supporting Python 3.7 and 3.8. Thank you!

Re: 2.6 and 3.3: I think of supported Python versions as a balance between development effort or needs, and the user's desire to not be forced to upgrade their Python version just for the sake of using Flask-User.

So my questions are:

• Are there significant developer benefits for dropping 2.6?
• Are there significant developer benefits for dropping 3.3?

Though I previously favored maintaining support for 2.6 and 3.3, I just learned that some of our developer tools (such as tox v3+), dropped support for Python 2.6 and 3.3. In fact, tox v3+ no longer works and produces an InvocationError (ironically triggered when trying to print a message that this python version is no longer supported).

Official support for Python 2.6 ended Oct 2013. pypistats show practically 0 downloads. Official support for Python 3.3 ended Sep 2017. pypistats doesn't seem to track this. Official support for Python 3.4 ended Mar 2019. pypistats shows an occasional download.

How about a middle ground for now: Drop support for Python 2.6 and 3.3 (because we no longer can auto-test these). Keep support for Python 3.4 just a little longer.

Thank you for your collaboration, @and-semakin !

[and-semakin]

I'm completely happy with this outcome! Thank you @lingthio!

[and-semakin]

Python 3.7 and 3.8 support added, Python 2.6 and 3.3 are dropped.