Bump postorius from 1.3.7 to 1.3.8 in /web (#589)

Bumps [postorius](https://gitlab.com/mailman/postorius) from 1.3.7 to 1.3.8.
- [Release notes](https://gitlab.com/mailman/postorius/tags)
- [Commits](https://gitlab.com/mailman/postorius/compare/1.3.7...1.3.8)

---
updated-dependencies:
- dependency-name: postorius
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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web/requirements.txt

@@ -1,4 +1,4 @@
 mailmanclient==3.3.5
-postorius==1.3.7
+postorius==1.3.8
 hyperkitty==1.3.7
 django-mailman3==1.3.8