fix `client.GetClassObject` type annotations and add tests

[Jun Komoda]

Remaining tasks for #456.

comtypes/comtypes/client/__init__.py

Lines 200 to 222 in 18dc2cc

 if TYPE_CHECKING: 
  
     @overload 
     def GetClassObject(progid, clsctx=None, pServerInfo=None): 
         # type: (_UnionT[str, CoClass, GUID], Optional[int], Optional[comtypes.COSERVERINFO]) -> hints.IClassFactory 
         pass 
  
     @overload 
     def GetClassObject(progid, clsctx=None, pServerInfo=None, interface=None): 
         # type: (_UnionT[str, CoClass, GUID], Optional[int], Optional[comtypes.COSERVERINFO], Optional[Type[_T_IUnknown]]) -> _T_IUnknown 
         pass 
  
  
 def GetClassObject(progid, clsctx=None, pServerInfo=None, interface=None): 
     # type: (_UnionT[str, CoClass, GUID], Optional[int], Optional[comtypes.COSERVERINFO], Optional[Type[IUnknown]]) -> IUnknown 
     """Create and return the class factory for a COM object. 
  
     'clsctx' specifies how to create the object, use the CLSCTX_... constants. 
     'pServerInfo', if used, must be a pointer to a comtypes.COSERVERINFO instance 
     'interface' may be used to request an interface other than IClassFactory 
     """ 
     clsid = GUID.from_progid(progid) 
     return comtypes.CoGetClassObject(clsid, clsctx, pServerInfo, interface) 

• remove if TYPE_CHECKING bridges, like #453.
• change type hints from comment annotations to inline annotations, like #453.
• add TestCase in test/test_client.py that call client.GetClassObject.
  • At least one of the tests should use a universal COM library such as those found in CI environments. Please do not use unittest.mock whenever possible.
• make sure that if passing Type[_T_IUnknown] for interface, the return value will be analyzed as _T_IUnknown by static type checkers.
• Before PR, please format the code by black (the maximum number of characters per line is 88, which is the default).

I hope someone who actually uses client.GetClassObject will PR along with the introduction of use cases, because I had not use this function.

[Jun Komoda]

This is related to #453.

Since comtypes.CoGetClassObject is also untested, we should avoid a situation where no tests without using unittest.mock.patch.