Add support for Python 3.9

[yakky]

Description

• Add support for Python 3.9
• Move haystack tox dependency only when testing with search features
• Drop python 3.5 from CI

References

Fix #657

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [x] changes file included (see docs)
• [x] Usage documentation added in case of new features
• [ ] Tests added

[codecov[bot]]

Codecov Report

Merging #658 (0c4d86e) into develop (6795ae5) will decrease coverage by 0.17%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #658      +/-   ##
===========================================
- Coverage    94.69%   94.51%   -0.18%     
===========================================
  Files           21       21              
  Lines         1112     1112              
  Branches       138      138              
===========================================
- Hits          1053     1051       -2     
- Misses          21       23       +2     
  Partials        38       38              

Flag	Coverage Δ
unittests	94.51% <ø> (-0.18%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
djangocms_blog/search_indexes.py	87.69% <0.00%> (-3.08%)	:arrow_down:

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[coveralls]

Coverage remained the same at 96.958% when pulling 0c4d86eda92e51678393127d0597e3a65ce559aa on feature/issue-657-python-3.9 into 6795ae536615ddaa8fedc6545603252d094c0e7d on develop.