Update pyyaml requirement from ~=5.3 to >=5.3,<7.0 (#1489)

Updates the requirements on [pyyaml](https://github.com/yaml/pyyaml) to permit the latest version.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/compare/5.3...6.0)

---
updated-dependencies:
- dependency-name: pyyaml
  dependency-type: direct:production
...

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setup.py

@@ -54,7 +54,7 @@ setup(
         "aiofiles>=0.4,<0.8",
         "janus>=0.4,<0.7",
         "asgi-csrf>=0.9",
-        "PyYAML~=5.3",
+        "PyYAML>=5.3,<7.0",
         "mergedeep>=1.1.1,<1.4.0",
         "itsdangerous>=1.1,<3.0",
         "python-baseconv==1.2.2",