Upgrade pyramid_mailer and repoze.sendmail

[nickstenning]

A long-standing bug in repoze.sendmail has now been fixed (repoze/repoze.sendmail#38), which means we can finally remove our version pin for this package.

[codecov-io]

Codecov Report

Merging #4482 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4482   +/-   ##
=======================================
  Coverage   94.58%   94.58%           
=======================================
  Files         354      354           
  Lines       18774    18774           
  Branches     1084     1084           
=======================================
  Hits        17757    17757           
+ Misses        908      905    -3     
- Partials      109      112    +3

Impacted Files	Coverage Δ
h/search/__init__.py	38.88% <0%> (ø)	⬆️
h/session.py	91.83% <0%> (ø)	⬆️
h/stats.py	83.33% <0%> (ø)	⬆️

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[chdorner]

LGTM. I tested it locally, but I'm not sure if the local email sending interceptor functionality actually tests any of these package upgrades. We might just have to put it on QA and see (although everything should hopefully be fine anyway).