add _contextvars for proper naming (#12981)

On 3.9 and lower, the contextvars types call themselves builtins.*
which we can't and won't match. This improves naming fidelity for
3.10 and newer.

stdlib/@tests/stubtest_allowlists/common.txt

@@ -382,7 +382,7 @@ codecs.CodecInfo.streamreader
 codecs.CodecInfo.streamwriter
 
 collections.UserList.sort  # Runtime has *args but will error if any are supplied
-contextvars.Context.__init__  # C signature is broader than what is actually accepted
+_?contextvars.Context.__init__  # C signature is broader than what is actually accepted
 
 # The Dialect properties are initialized as None in Dialect but their values are enforced in _Dialect
 csv.Dialect.delimiter

stdlib/@tests/stubtest_allowlists/py311.txt

@@ -144,7 +144,6 @@ weakref.ProxyType.__reversed__  # Doesn't really exist
 # C signature is broader than what is actually accepted
 ast.ExtSlice.__new__
 ast.Index.__new__
-contextvars.Context.__init__
 
 # Undocumented implementation details
 cgi.FieldStorage.bufsize

stdlib/@tests/stubtest_allowlists/py312.txt

@@ -149,7 +149,6 @@ sys.last_exc
 # C signature is broader than what is actually accepted
 ast.ExtSlice.__new__
 ast.Index.__new__
-contextvars.Context.__init__
 
 # Treated an alias of a typing class in the stubs,
 # they are generic to type checkers anyway.

stdlib/@tests/stubtest_allowlists/py313.txt

@@ -125,7 +125,6 @@ sys.last_exc
 # C signature is broader than what is actually accepted
 ast.ExtSlice.__new__
 ast.Index.__new__
-contextvars.Context.__init__
 
 # Treated an alias of a typing class in the stubs,
 # they are generic to type checkers anyway.

stdlib/@tests/stubtest_allowlists/py38.txt

@@ -113,7 +113,7 @@ tkinter.Tcl
 tkinter.Tk.__init__
 
 # Exists at runtime, but missing from stubs
-contextvars.ContextVar.__class_getitem__
+_?contextvars.ContextVar.__class_getitem__
 datetime.datetime_CAPI
 dummy_threading.ExceptHookArgs
 dummy_threading.Lock