[3.9] bpo-37116: Use PEP 570 syntax for positional-only parameters. (GH-12620)

Turn deprecation warnings added in 3.8 into TypeError.

Lib/concurrent/futures/_base.py

@@ -547,7 +547,7 @@ class Future(object):
 class Executor(object):
     """This is an abstract base class for concrete asynchronous executors."""
 
-    def submit(*args, **kwargs):
+    def submit(self, fn, /, *args, **kwargs):
         """Submits a callable to be executed with the given arguments.
 
         Schedules the callable to be executed as fn(*args, **kwargs) and returns
@@ -556,21 +556,7 @@ class Executor(object):
         Returns:
             A Future representing the given call.
         """
-        if len(args) >= 2:
-            pass
-        elif not args:
-            raise TypeError("descriptor 'submit' of 'Executor' object "
-                            "needs an argument")
-        elif 'fn' in kwargs:
-            import warnings
-            warnings.warn("Passing 'fn' as keyword argument is deprecated",
-                          DeprecationWarning, stacklevel=2)
-        else:
-            raise TypeError('submit expected at least 1 positional argument, '
-                            'got %d' % (len(args)-1))
-
         raise NotImplementedError()
-    submit.__text_signature__ = '($self, fn, /, *args, **kwargs)'
 
     def map(self, fn, *iterables, timeout=None, chunksize=1):
         """Returns an iterator equivalent to map(fn, iter).