Bump stem from 1.8.0 to 1.8.1 (#689)

Bumps [stem](https://github.com/torproject/stem) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/torproject/stem/releases)
- [Commits](https://github.com/torproject/stem/compare/1.8.0...1.8.1)

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

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requirements.txt

@@ -25,7 +25,7 @@ requests-futures==1.0.0
 six==1.16.0
 socid-extractor>=0.0.21
 soupsieve==2.3.2.post1
-stem==1.8.0
+stem==1.8.1
 torrequest==0.1.0
 tqdm==4.64.1
 typing-extensions==4.3.0