Remove unneeded dependencies

[s-weigand]

Atm the CI tests fail since the requirements_dev.txt contains dependecies which are fixed, outdated and often not needed (i.e. cryptography which breaks the build).

This PR:

• Drops unnesessary dependecys
• Releases the fixed version of dependencies which aren't runtime dependencies
• Drops deprecated python versions
• Adds flake8 testing to the tests on travis-ci

Note: The build is breaking due to the failing flake8 test. I didn't fix this since most project don't like white space or formation changes. But if you want I can format the code pep8 conform as done here.

Since this PR releases fixed versions this closes #90 #89 #84 #80 #79

[s-weigand]

Closed the PR since this package isn't actively developed anymore (see #91).