Fix pip issue & Add support to custom endpoint_url, region_name and signature_version

[M0dM]

Trying to fix #42

I could not test it...
I have a strange issue on master :

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/home/benoit/.local/share/virtualenvs/dxf_geom2d/src/kev/setup.py", line 2, in <module>
        from pip.req import parse_requirements
    ModuleNotFoundError: No module named 'pip.req'

Edit: pip issue (#44) Problem fixed by commit 613e2c7

I need also region_name argument to be set, I will try to add it as well in a new commit...
This will be easier as this attribute can be set directly when instanciating the session.

Edit: region_name and signature_version are also usefull to make kev work with minio

[M0dM]

Everything seems to work well on my side.
As tests are completely broken, could you please test on your side and merge this if you find it legit.

Regards,