bpo-43680: _pyio.open() becomes a static method (GH-25354)

The Python _pyio.open() function becomes a static method to behave as
io.open() built-in function: don't become a bound method when stored
as a class variable. It becomes possible since static methods are now
callable in Python 3.10. Moreover, _pyio.OpenWrapper becomes a simple
alias to _pyio.open.

init_set_builtins_open() now sets builtins.open to io.open, rather
than setting it to io.OpenWrapper, since OpenWrapper is now an alias
to open in the io and _pyio modules.

Lib/_pyio.py

@@ -63,6 +63,13 @@ def text_encoding(encoding, stacklevel=2):
     return encoding
 
 
+# Wrapper for builtins.open
+#
+# Trick so that open() won't become a bound method when stored
+# as a class variable (as dbm.dumb does).
+#
+# See init_set_builtins_open() in Python/pylifecycle.c.
+@staticmethod
 def open(file, mode="r", buffering=-1, encoding=None, errors=None,
          newline=None, closefd=True, opener=None):
 
@@ -313,18 +320,9 @@ class DocDescriptor:
                  "errors=None, newline=None, closefd=True)\n\n" +
             open.__doc__)
 
-class OpenWrapper:
-    """Wrapper for builtins.open
 
-    Trick so that open won't become a bound method when stored
-    as a class variable (as dbm.dumb does).
-
-    See initstdio() in Python/pylifecycle.c.
-    """
-    __doc__ = DocDescriptor()
-
-    def __new__(cls, *args, **kwargs):
-        return open(*args, **kwargs)
+# bpo-43680: Alias to open() kept for backward compatibility
+OpenWrapper = open
 
 
 # In normal operation, both `UnsupportedOperation`s should be bound to the