Minor improvements to stubtest_third_party.py (#10605)

(1) Give a nicer error message if you try to run stubtest on non-existent stubs.
(2) Print the Python version to the terminal if there's an error, as well as the output of pip freeze.

tests/parse_metadata.py

@@ -20,6 +20,7 @@ from packaging.version import Version
 from utils import cache
 
 __all__ = [
+    "NoSuchStubError",
     "StubMetadata",
     "PackageDependencies",
     "StubtestSettings",
@@ -160,6 +161,10 @@ _KNOWN_METADATA_TOOL_FIELDS: Final = {
 _DIST_NAME_RE: Final = re.compile(r"^[a-z0-9]([a-z0-9._-]*[a-z0-9])?$", re.IGNORECASE)
 
 
+class NoSuchStubError(ValueError):
+    """Raise NoSuchStubError to indicate that a stubs/{distribution} directory doesn't exist"""
+
+
 @cache
 def read_metadata(distribution: str) -> StubMetadata:
     """Return an object describing the metadata of a stub as given in the METADATA.toml file.
@@ -169,8 +174,11 @@ def read_metadata(distribution: str) -> StubMetadata:
     Use `read_dependencies` if you need to parse the dependencies
     given in the `requires` field, for example.
     """
-    with Path("stubs", distribution, "METADATA.toml").open("rb") as f:
-        data: dict[str, object] = tomli.load(f)
+    try:
+        with Path("stubs", distribution, "METADATA.toml").open("rb") as f:
+            data: dict[str, object] = tomli.load(f)
+    except FileNotFoundError:
+        raise NoSuchStubError(f"Typeshed has no stubs for {distribution!r}!") from None
 
     unknown_metadata_fields = data.keys() - _KNOWN_METADATA_FIELDS
     assert not unknown_metadata_fields, f"Unexpected keys in METADATA.toml for {distribution!r}: {unknown_metadata_fields}"