PEP 484: Clarify definition of AnyStr (#744)

The definition of AnyStr as given makes sense only in Python 3, as observed by @srittau. To prevent confusion, this PR changes the definition to use Text instead of str.

I also changed the two places where AnyStr was defined as an example, and made a few other related adjustments:
- Remove mention of None as modifying the type, which is not true as of #689.
- Move Text up in the list of convenience definition because we now need it for the definition of AnyStr.
- Modify definition of Optional to use `None` instead of `type(None)`, because the latter is not a valid static type.

Author: JelleZijlstra

pep-0484.txt

@@ -312,9 +312,9 @@ example, we can define a type variable that ranges over just ``str`` and
 ``bytes``. By default, a type variable ranges over all possible types.
 Example of constraining a type variable::
 
-  from typing import TypeVar
+  from typing import TypeVar, Text
 
-  AnyStr = TypeVar('AnyStr', str, bytes)
+  AnyStr = TypeVar('AnyStr', Text, bytes)
 
   def concat(x: AnyStr, y: AnyStr) -> AnyStr:
       return x + y
@@ -1357,8 +1357,7 @@ without specifying the actual default value.  For example::
   def foo(x: AnyStr, y: AnyStr = ...) -> AnyStr: ...
 
 What should the default value look like?  Any of the options ``""``,
-``b""`` or ``None`` fails to satisfy the type constraint (actually,
-``None`` will *modify* the type to become ``Optional[AnyStr]``).
+``b""`` or ``None`` fails to satisfy the type constraint.
 
 In such cases the default value may be specified as a literal
 ellipsis, i.e. the above example is literally what you would write.
@@ -1785,9 +1784,9 @@ A constrained ``TypeVar`` type can often be used instead of using the
 ``@overload`` decorator.  For example, the definitions of ``concat1``
 and ``concat2`` in this stub file are equivalent::
 
-  from typing import TypeVar
+  from typing import TypeVar, Text
 
-  AnyStr = TypeVar('AnyStr', str, bytes)
+  AnyStr = TypeVar('AnyStr', Text, bytes)
 
   def concat1(x: AnyStr, y: AnyStr) -> AnyStr: ...
 
@@ -2024,12 +2023,12 @@ A few one-off types are defined that test for single special methods
 
 Convenience definitions:
 
-* Optional, defined by ``Optional[t] == Union[t, type(None)]``
-
-* AnyStr, defined as ``TypeVar('AnyStr', str, bytes)``
+* Optional, defined by ``Optional[t] == Union[t, None]``
 
 * Text, a simple alias for ``str`` in Python 3, for ``unicode`` in Python 2
 
+* AnyStr, defined as ``TypeVar('AnyStr', Text, bytes)``
+
 * NamedTuple, used as
   ``NamedTuple(type_name, [(field_name, field_type), ...])``
   and equivalent to