Bring back CentOS 7

[akuzminsky]

• Bring back CentOS 7
• Configure bumpversion for doc files
• Bump version: 3.2.1 → 3.3.0

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (127f446) 52.77% compared to head (dd993e5) 52.77%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #410   +/-   ##
=======================================
  Coverage   52.77%   52.77%           
=======================================
  Files          65       65           
  Lines        3807     3807           
=======================================
  Hits         2009     2009           
  Misses       1798     1798           

Files Changed	Coverage Δ
twindb_backup/__init__.py	98.33% <100.00%> (ø)

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