Make (GS,LS)Index an indexable type in runtime (#769)

Following 5426ed9, users found themselves having to write some rather unergonomic code to reconcile Python's and a type-checker's perspectives:

    if TYPE_CHECKING:
        _MyModelGSI = GlobalSecondaryIndex["MyModel"]
    else:
        _MyModelGSI = GlobalSecondaryIndex

    class MyModelUserIdGSI(_MyModelGSI):
        """A GSI for looking up by user ID."""
       ...

    class MyModelEmailGSI(_MyModelGSI):
        """A GSI for looking up by email."""
       ...

This addresses it by allowing users to rewrite this as:

    class MyModelUserIdGSI(GlobalSecondaryIndex["MyModel"]):
        """A GSI for looking up by user ID."""
       ...

    class MyModelEmailGSI(GlobalSecondaryIndex["MyModel"]):
        """A GSI for looking up by email."""
       ...

Since we maintain Python 2 compatibility, we cannot assume the typing module is available so we're making a mock Generic class instead.

docs/release_notes.rst

@@ -1,6 +1,11 @@
 Release Notes
 =============
 
+v4.3.2
+----------
+
+* Fix discrepancy between runtime and type-checker's perspective of Index and derived types
+
 v4.3.1
 ----------
 

pynamodb/__init__.py

@@ -7,4 +7,4 @@
 """
 __author__ = 'Jharrod LaFon'
 __license__ = 'MIT'
-__version__ = '4.3.1'
+__version__ = '4.3.2'

pynamodb/_compat.py

@@ -7,6 +7,12 @@
     from inspect import getfullargspec
 
 
+class FakeGenericMeta(type):
+    """Poor man's Generic[T] that doesn't depend on typing. The real generics are in the type stubs."""
+    def __getitem__(self, item):
+        return self
+
+
 def load_module(name, path):
     """Load module using the Python version compatible function."""
     if sys.version_info >= (3, 3):
@@ -23,4 +29,5 @@ def load_module(name, path):
         from imp import load_source
         return load_source(name, path)
 
-__all__ = ('getfullargspec', 'load_module')
+
+__all__ = ('getfullargspec', 'FakeGenericMeta', 'load_module')

pynamodb/indexes.py

@@ -3,6 +3,7 @@
 """
 from inspect import getmembers
 
+from pynamodb._compat import FakeGenericMeta
 from pynamodb.constants import (
     INCLUDE, ALL, KEYS_ONLY, ATTR_NAME, ATTR_TYPE, KEY_TYPE, ATTR_TYPE_MAP, KEY_SCHEMA,
     ATTR_DEFINITIONS, META_CLASS_NAME
@@ -13,7 +14,7 @@
 from six import with_metaclass
 
 
-class IndexMeta(type):
+class IndexMeta(FakeGenericMeta):
     """
     Index meta class