Allow three-component third-party versions (#5995)

Closes: #5992

CONTRIBUTING.md

@@ -96,8 +96,10 @@ The metadata file describes the stubs package using the
 supported:
 
 * `version`: The latest version of the library that the stubs support.
-  Note that only two most significant version levels are supported
-  (i.e. only single dot). When the stubs are updated to a newer version
+  For libraries that reflect API changes in the version number only
+  the parts indicating the API level should be specified. In the case
+  of [Semantic Versioning](https://semver.org/) that is the first two
+  components. When the stubs are updated to a newer version
   of the library, the version of the stub should be bumped (note that
   previous versions are still available on PyPI). Some legacy stubs are
   marked with version `0.1`, indicating that their supported version is

tests/check_consistent.py

@@ -169,9 +169,7 @@ def check_metadata():
         assert "version" in data, f"Missing version for {distribution}"
         version = data["version"]
         msg = f"Unsupported Python version {version}"
-        assert version.count(".") == 1, msg
-        major, minor = version.split(".")
-        assert major.isdigit() and minor.isdigit(), msg
+        assert re.match(r"^\d+\.\d+(\.\d+)?$", version), msg
         for key in data:
             assert key in metadata_keys, f"Unexpected key {key} for {distribution}"
         assert isinstance(data.get("python2", False), bool), f"Invalid python2 value for {distribution}"