Make os.path.join() first parameter pos-only (#6812)

tests/stubtest_allowlists/py310.txt

@@ -115,6 +115,10 @@ contextlib.AbstractContextManager.__exit__
 io.IncrementalNewlineDecoder.setstate
 typing.SupportsRound.__round__
 types.DynamicClassAttribute..*  # In the stub we pretend it's an alias for property, but it has positional-only differences
+# These three have a pos-or-keyword first parameter at runtime, but deliberately have a pos-only first parameter in the stub. #6812
+posixpath.join
+ntpath.join
+os.path.join
 
 # These enums derive from (str, Enum). See comment in py3_common.txt
 pstats.SortKey.__new__