Fix postgres drivers detection

[yakky]

• add support for 'short' postgres driver (available when next dj_database_url is released)
• don't pin psycopg2<2.8 in django 2.2+

Fix #355 Fix #354

[coveralls]

Coverage increased (+0.2%) to 95.853% when pulling 5a13b0ae73b4089aa406367998317c42efbb38f6 on hotfix/db-drivers into 4ce314c2c9c8356a6b648b1f150750c33657e37b on develop.

[coveralls]

Coverage increased (+0.01%) to 95.645% when pulling 5a13b0ae73b4089aa406367998317c42efbb38f6 on hotfix/db-drivers into 4ce314c2c9c8356a6b648b1f150750c33657e37b on develop.

[codecov[bot]]

Codecov Report

Merging #358 into develop will increase coverage by 0.02%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #358      +/-   ##
===========================================
+ Coverage    91.53%   91.56%   +0.02%     
===========================================
  Files            7        7              
  Lines          638      640       +2     
  Branches       131      132       +1     
===========================================
+ Hits           584      586       +2     
  Misses          24       24              
  Partials        30       30

Impacted Files	Coverage Δ
djangocms_installer/config/data.py	100% <ø> (ø)	:arrow_up:
djangocms_installer/config/__init__.py	92.39% <100%> (+0.08%)	:arrow_up:

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