build(deps): bump plexapi-backport from 4.15.4 to 4.15.6 (#61)

Bumps [plexapi-backport](https://github.com/LizardByte/python-plexapi-backport) from 4.15.4 to 4.15.6.
- [Commits](https://github.com/LizardByte/python-plexapi-backport/commits)

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

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

requirements.txt

@@ -3,7 +3,7 @@
 flask==1.1.4;python_version<"3"
 flask-babel==1.0.0;python_version<"3"
 future==0.18.3
-plexapi-backport==4.15.4  # custom python-plexapi supporting python 2.7
+plexapi-backport==4.15.6  # custom python-plexapi supporting python 2.7
 polib==1.2.0;python_version<"3"
 requests==2.27.1;python_version<"3"
 typing==3.10.0.0