perform python version checks in cmd/main ...

[jgrewe]

... will raise a runtime error if python version is below 3.6
... will create argparse.add_subparsers without requires=True for python 3.6
... and will use it for python >= 3.7

fixes #518

[coveralls]

Coverage decreased (-0.04%) to 78.998% when pulling ab6bb7e on jgrewe:cmd_required into f2c0cd6 on G-Node:master.

[codecov-commenter]

Codecov Report

Merging #519 (ab6bb7e) into master (f2c0cd6) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
- Coverage   78.77%   78.73%   -0.05%     
==========================================
  Files          59       59              
  Lines        9386     9391       +5     
==========================================
  Hits         7394     7394              
- Misses       1992     1997       +5     

Impacted Files	Coverage Δ
nixio/cmd/main.py	0.00% <0.00%> (ø)

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