Compatibility with Python 3.9

[honzajavorek]

I'm getting following warnings:

/Users/honza/.local/share/virtualenvs/python.cz-s2NxYiyq/lib/python3.7/site-packages/flask_frozen/__init__.py:29
  /Users/honza/.local/share/virtualenvs/python.cz-s2NxYiyq/lib/python3.7/site-packages/flask_frozen/__init__.py:29:
  DeprecationWarning: Using or importing the ABCs from 'collections' instead
  of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    from collections import Mapping, namedtuple

tests/views_test.py::test_get_involved_cs_renders_ordinary_issue
  /Users/honza/.local/share/virtualenvs/python.cz-s2NxYiyq/lib/python3.7/site-packages/ics/icalendar.py:59:
  DeprecationWarning: Using or importing the ABCs from 'collections' instead
  of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    elif isinstance(imports, collections.Iterable):

[tswast]

Thanks for reporting. PRs welcome!

[hroncok]

Is preserving Python 2 compatibility required?

[tswast]

I'd lean towards keeping in line with the latest version of Flask, but they do still support Python 2.7. https://github.com/pallets/flask/blob/64ba43411fbfb2e435455b0df2453d7809b7c5a8/setup.py

I think it's probably worth dropping 2.7 so long as Python 3.5 is still supported.

[honzajavorek]

FYI, I'm getting failures with Travis CI trying to install Py 2.x versions under #97. But it also fails to install the PyPy versions, so I'm unsure what's the issue.

[honzajavorek]

Added a tested solution here: #98

[jayaddison]

@tswast @honzajavorek Do you know if there are any plans to release a py3.9-compatible release of Frozen-Flask to PyPi?

[honzajavorek]

I think it's mainly a matter of whether there's a contribution adding the support.

I'm unsure whether it's a big task or not, given we still support 2.7. If supporting 3.9 is easier (and cleaner) without 2.7, there's probably no big harm in watching when Flask drops 2.7 and only then add support for 3.9. I don't have any strong opinions about this though.

[jayaddison]

Ok, that makes sense. The good news: running the Flask-Frozen test suite under Python 3.9, it seems to pass as-is. I'll open a pull request to enable CI in a moment.

[honzajavorek]

OK so previously I've replied from my phone and I missed the fact this is closed already. I recalled I wanted to tackle this problem in #98, which even mentions 3.9 and I think it tests the issues and fixes them, but for some reason, I forgot to add it to the testing matrix 😄 So that probably explains why it passes as-is.

[jayaddison]

No probs, thanks! :)