build(deps): bump libmozevent from 1.1.20 to 1.1.21 in /events (#1677)

Bumps [libmozevent](https://github.com/mozilla/libmozevent) from 1.1.20 to 1.1.21.
- [Release notes](https://github.com/mozilla/libmozevent/releases)
- [Commits](mozilla/libmozevent@1.1.20...1.1.21)

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

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Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

events/requirements.txt

@@ -1,2 +1,2 @@
 -e ../tools #egg=code-coverage-tools
-libmozevent==1.1.20
+libmozevent==1.1.21