Bump plexapi from 4.12.1 to 4.13.1 (#1888)

Bumps [plexapi](https://github.com/pkkid/python-plexapi) from 4.12.1 to 4.13.1.
- [Release notes](https://github.com/pkkid/python-plexapi/releases)
- [Commits](https://github.com/pkkid/python-plexapi/compare/4.12.1...4.13.1)

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

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[skip ci]

lib/plexapi/config.py

@@ -29,7 +29,7 @@ class PlexConfig(ConfigParser):
         """
         try:
             # First: check environment variable is set
-            envkey = 'PLEXAPI_%s' % key.upper().replace('.', '_')
+            envkey = f"PLEXAPI_{key.upper().replace('.', '_')}"
             value = os.environ.get(envkey)
             if value is None:
                 # Second: check the config file has attr