Prepare support python 3.7, 3.8

[sh1ng]

I prefer to merge it when tests for python 3.6 are passed as keeping it longer will slowdown CI because of docker cache misses.

• fixed swig wrapper
• LLVM 7.1 as we use quite old LLVM 6.0 that is not supported by python 3.7+
• pylint

I'm going to add CI configuration for python 3.7 and 3.8 in a separate PR

[sh1ng]

I was using default value 3.7 to tests correctness while CI script stays unchanged. Will remove it at all.