Make setuptools dependency optional

[edwardpeek-crown-public]

Fallback assignment of model_utils.__version__ = None if pkg_resources can not be imported due to setuptools not being installed

Problem

Fixes #506

Solution

Wrap pkg_resource import in a try block as done in other projects like pytz to make dependency optional.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[codecov]

Codecov Report

Merging #507 (f4f2704) into master (903f640) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #507   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files           6        6           
  Lines         825      825           
=======================================
  Hits          785      785           
  Misses         40       40           

Impacted Files	Coverage Δ
model_utils/__init__.py	100.00% <100.00%> (ø)

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[foarsitter]

#545 replaced pkg_resource with importlib