Update classifiers

[hugovk]

Support for EOL Python 2.6 and 3.3 were removed in ccaff6c#diff-354f30a63fb0907d4ad57269548329e3 and 1c7691b#diff-354f30a63fb0907d4ad57269548329e3.

Update the classifiers to match.

This also modernises some code which is possible in 2.7/3.4+.

[codecov-io]

Codecov Report

Merging #87 into master will not change coverage.
The diff coverage is 100%.

@@ Coverage Diff @@## master #87 +/- ##
=======================================
Coverage 93.62% 93.62% =======================================
Files 13 13 Lines 1004 1004 =======================================
Hits 940 940 Misses 64 64

Impacted Files	Coverage Δ
jose/jwk.py	95.12% <100%> (ø)	⬆️
jose/constants.py	100% <100%> (ø)	⬆️

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[hugovk]

And here's the pip installs for python-jose from PyPI for last month:

python_version	percent	download_count
2.7	90.46%	195,778
3.6	5.68%	12,298
3.5	2.27%	4,904
3.4	1.57%	3,402
3.7	0.01%	19
2.6	0.01%	18

Source: pypinfo --start-date -31 --end-date -2 --percent --pip --markdown python-jose pyversion

[zejn]

Looks good to me.

[mpdavis]

lgtm