re-revert towncrier to 18.5.0

[tardyp]

see #4259

twisted/towncrier#121

[codecov]

Codecov Report

Merging #4305 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4305      +/-   ##
==========================================
+ Coverage   88.53%   88.54%   +<.01%     
==========================================
  Files         329      329              
  Lines       34616    34616              
==========================================
+ Hits        30649    30650       +1     
+ Misses       3967     3966       -1

Impacted Files	Coverage Δ
master/buildbot/locks.py	95.97% <0%> (+0.67%)	⬆️

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