Consider dropping advertised support for CPython 2.6 and 3.3

[mattsb42-aws]

As we note in the docs[1], there is nothing in this library that should require any newer CPython version than 2.6, when the io module was added to the standard library. In theory it should also work with old versions of CPython 3 as well, though changes introduced in 3.3 might be required for #22 to work.

Because of the above, we have tried to maintain testing in our CI for 2.6 and 3.3 (iirc I originally tried to get 3.0, 3.1, and 3.2 working but couldn't get the CI to work).

We are now encountering issues with running our CI with 3.3, and 2.6 will likely continue to become more and more unsupported.

CPython 2.6 last release was 2013-10-29.
https://www.python.org/dev/peps/pep-0361/

CPython 3.3 went EOL on 2017-09-29.
https://www.python.org/dev/peps/pep-0398/

I want to collect some download numbers before making the decision, but I think it might be reasonable for us to just drop advertised support for pre-2.7 and pre-3.4.

[1] https://base64io-python.readthedocs.io/en/latest/#getting-started

[mattsb42-aws]

Based on the current results from pypistats for year to date, I am inclined to say that dropping advertised support for 3.3 and 2.6 is reasonable. It looks like we have had exactly 0 3.3 downloads, and only 3 2.6 downloads.

category	percent	downloads
2.7	95.81%	394,684
3.6	2.42%	9,980
3.7	0.90%	3,702
3.5	0.56%	2,292
3.4	0.20%	822
null	0.10%	427
3.8	0.01%	27
2.6	0.00%	3
Total		411,937

[mattsb42-aws]

To be clear: we will not be blocking downloads for these versions, we will only stop testing them in our CI and remove them from the advertised list of supported versions.