Fix onerror callback type hinting, improve style

The onerror parameter of shutil.rmtree receives a 3-tuple like what
sys.exc_info() gives, not a string. Since we are not using that
parameter anyway, I've fixed it in the onerror function defined
in git.util.rmtree by changing it to Any rather than hinting it
narrowly.

I've also renamed the parameters so the names are based on those
that are documented in the shutil.rmtree documentation. The names
are not part of the function's interface (this follows both from
the documentation and the typeshed hints) but using those names may
make it easier to understand the function.

- func is renamed to function.

- path remains path.

- exc_info is renamed to _excinfo. This parameter is documented as
  excinfo, but I've prepended an underscore to signifity that it is
  unused.

These changes are to a local function and non-breaking.

Finally, although not all type checkers will flag this as an error
automatically, the git.util.rmtree function, like the shutil.rmtree
function it calls, is conceptually void, so it should not have any
return statements with operands. Because the return statement
appeared at the end, I've removed "return".

git/util.py

@@ -182,20 +182,20 @@ def rmtree(path: PathLike) -> None:
     :note: we use shutil rmtree but adjust its behaviour to see whether files that
         couldn't be deleted are read-only. Windows will not remove them in that case"""
 
-    def onerror(func: Callable, path: PathLike, exc_info: str) -> None:
+    def onerror(function: Callable, path: PathLike, _excinfo: Any) -> None:
         # Is the error an access error ?
         os.chmod(path, stat.S_IWUSR)
 
         try:
-            func(path)  # Will scream if still not possible to delete.
+            function(path)  # Will scream if still not possible to delete.
         except PermissionError as ex:
             if HIDE_WINDOWS_KNOWN_ERRORS:
                 from unittest import SkipTest
 
                 raise SkipTest(f"FIXME: fails with: PermissionError\n  {ex}") from ex
             raise
 
-    return shutil.rmtree(path, False, onerror)
+    shutil.rmtree(path, False, onerror)
 
 
 def rmfile(path: PathLike) -> None: