Add 'python_requires' to setup.py to specify python support

[djhoese]

Add what versions of python are supported by satpy to the package metadata (setup.py). This should help us in the future when we drop certain versions of python and will stop users' pip environments installing newer versions of satpy on older versions of python.

• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff

[codecov-io]

Codecov Report

Merging #256 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #256   +/-   ##
========================================
  Coverage    64.06%   64.06%           
========================================
  Files          112      112           
  Lines        13863    13863           
========================================
  Hits          8882     8882           
  Misses        4981     4981

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[coveralls]

Coverage remained the same at 64.07% when pulling 8366964 on davidh-ssec:bugfix-python-requires into 6501a07 on pytroll:develop.

[coveralls]

Coverage remained the same at 64.07% when pulling 8366964 on davidh-ssec:bugfix-python-requires into 6501a07 on pytroll:develop.

[coveralls]

Coverage remained the same at 64.07% when pulling 8366964 on davidh-ssec:bugfix-python-requires into 6501a07 on pytroll:develop.