Support Python >=3.6

[benspaulding]

I took a shot at solving #26 and supporting Python >=3.6. I also added some instructions on how to develop on the package. Take it all, or pick and choose if there is only some of this you want.

Thanks for creating DjHTML!

[benspaulding]

I was messing with the version in setup.cfg for some reason and should not have. I will push up a new commit without that.

[JaapJoris]

Dude, you're awesome! I had no idea how to automate testing against multiple Python interpreters and I had never heard of Nox. This seems like exactly the right tool for the job!

Using the new Readme instructions, I ran the following commands:

$ pip install nox
$ nox
nox > Running session lint-isort
nox > Creating virtual environment (virtualenv) using python3 in .nox/lint-isort
nox > python -m pip install isort==5.8.0
nox > python -m isort .
Skipped 2 files
nox > Session lint-isort was successful.
nox > Running session lint-black
nox > Creating virtual environment (virtualenv) using python3 in .nox/lint-black
nox > python -m pip install black==21.5b0
nox > python -m black .
All done! ✨ 🍰 ✨
9 files left unchanged.
nox > Session lint-black was successful.
nox > Running session lint-flake8
nox > Creating virtual environment (virtualenv) using python3 in .nox/lint-flake8
nox > python -m pip install flake8==3.9.1
nox > python -m flake8 .
nox > Session lint-flake8 was successful.
nox > Running session tests-3.6
nox > Session tests-3.6 skipped: Python interpreter 3.6 not found.
nox > Running session tests-3.7
nox > Session tests-3.7 skipped: Python interpreter 3.7 not found.
nox > Running session tests-3.8
nox > Session tests-3.8 skipped: Python interpreter 3.8 not found.
nox > Running session tests-3.9
nox > Creating virtual environment (virtualenv) using python3.9 in .nox/tests-3-9
nox > python -m unittest discover -v
test_available_files (tests.test_suite.TestSuite)
Loop over all the files in the suite directory and compare the ... ok

----------------------------------------------------------------------
Ran 1 test in 0.007s

OK
nox > Session tests-3.9 was successful.
nox > Ran multiple sessions:
nox > * lint-isort: success
nox > * lint-black: success
nox > * lint-flake8: success
nox > * tests-3.6: skipped
nox > * tests-3.7: skipped
nox > * tests-3.8: skipped
nox > * tests-3.9: success

As you can see, I was unable to confirm that your PR actually improves compatibility with older Python versions, since none of them could be found on my system 😂

However, I will still happily accept your PR because I'm confident that these tests pass on your system. Thank you very much, and I hope you don't mind I'll consult you whenever I have compatibility issues in the future 🙏