CPython 3.4 support

[mattsb42-aws]

In the next release, pyca/cryptography is dropping support for CPython 3.4[1]. This library is what initially set our lower bound for CPython version support, and if they are dropping support for 3.4 we should consider dropping that support as well because we rely so heavily on them.

Based on our PyPI download stats this year and the last month, I think that this is a low-risk course of action.

Date range: 2019-01-01 - 2019-12-31

category	percent	downloads
2.7	62.88%	795,886
3.6	33.11%	419,118
3.7	1.87%	23,614
3.5	1.74%	22,010
3.4	0.22%	2,726
null	0.14%	1,831
3.8	0.04%	503
2.6	0.00%	5
Total		1,265,693

Date range: 2019-11-01 - 2019-11-30

category	percent	downloads
2.7	68.48%	157,540
3.6	27.56%	63,417
3.7	2.36%	5,425
3.5	1.27%	2,919
3.4	0.14%	325
null	0.14%	319
3.8	0.05%	122
Total		230,067

[1] pyca/cryptography#5087

[mattsb42-aws]

I've run into problems with this while adding the AWS KMS keyring because I pulled in moto for testing and it pull in pyyaml, which dropped support for 3.4 in 2019-12.

Because of this and the fact that keyrings will constitute a Y release anyway, I'm going to use this as an opportunity to drop 3.4 testing.