Fix and modernize build

[jochen-ott-by]

• fix an issue with azure-storage-blob 12 and python 2: the returned blob names are now of type unicode
• use pycodestyle instead of the deprecated pep8
• apply pycodestyle not only to the code in tests/, but to the actual library code. Note that some fixes were necessary to make it pass, I kept them in a separate commit for easier review
• removed tests for python 3.4, added python 3.7 and python 3.8
• run the azure tests in travis using azurite. This way, we don't need credentials for a real azure blob store account for the tests, which gives much better (earlier) test feedback that was missing earlier.

[coveralls]

Coverage increased (+11.09%) to 89.779% when pulling 56bf1fa92f6a2f7692b61c18944bf2cefdbcd8d9 on jochen-ott-jdas:fix-and-modernize-build into 994f0a86f571e25c2f8af5c6d61103b3a0b9b6c4 on mbr:master.