Update support of Python and Django versions

[hramezani]

Problem

Explain the problem you are fixing (add the link to the related issue(s), if any).

Solution

Explain the solution that has been implemented, and what has been changed.

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 #532 (85b656e) into master (d699745) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #532   +/-   ##
=======================================
  Coverage   95.12%   95.12%           
=======================================
  Files           6        6           
  Lines         820      820           
=======================================
  Hits          780      780           
  Misses         40       40           

Impacted Files	Coverage Δ
model_utils/managers.py	98.08% <100.00%> (ø)

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[charettes]

Thanks for the version bumps @hramezani!