Fix email typing errors (#288)

* Change (Py3) email message payload type

The docstring of the Message get_payload method indicates that the
possible return types include None and bytes. Additionally the
set_payload method accepts bytes. Therefore I've changed the
PayloadType to include bytes and the get_payload return type to be an
Optional PayloadType.

* Change email (Py3) message_from_file IO types

Instead of using TextIO and BinaryIO use IO[str] and IO[bytes]
respectively to match the type returned by the open builtin. This
follows the recommendations in #283.

The error that prompts this is reproduced via:
```
from email import message_from_file

with open('email') as file_:
    email = message_from_file(file_)
```
Argument 1 to "message_from_file" has incompatible type IO[Any]; expected "TextIO"

stdlib/3/email/__init__.pyi

@@ -1,6 +1,6 @@
 # Stubs for email (Python 3.4)
 
-from typing import Callable, Optional, BinaryIO, TextIO
+from typing import Callable, Optional, IO
 import sys
 from email.message import Message, Policy
 
@@ -9,9 +9,9 @@ if sys.version_info >= (3, 3):
                             policy: Policy = ...) -> Message: ...
     def message_from_bytes(s: bytes, _class: Callable[[], Message] = ..., *,
                            policy: Policy = ...) -> Message: ...
-    def message_from_file(fp: TextIO, _class: Callable[[], Message] = ..., *,
+    def message_from_file(fp: IO[str], _class: Callable[[], Message] = ..., *,
                            policy: Policy = ...) -> Message: ...
-    def message_from_binary_file(fp: BinaryIO,
+    def message_from_binary_file(fp: IO[bytes],
                                  _class: Callable[[], Message] = ..., *,
                                  policy: Policy = ...) -> Message: ...
 elif sys.version_info >= (3, 2):
@@ -21,10 +21,10 @@ elif sys.version_info >= (3, 2):
     def message_from_bytes(s: bytes,  # type: ignore
                            _class: Callable[[], Message] = ..., *,
                            strict: Optional[bool] = ...) -> Message: ...
-    def message_from_file(fp: TextIO,  # type: ignore
+    def message_from_file(fp: IO[str],  # type: ignore
                           _class: Callable[[], Message] = ..., *,
                           strict: Optional[bool] = ...) -> Message: ...
-    def message_from_binary_file(fp: BinaryIO,  # type: ignore
+    def message_from_binary_file(fp: IO[bytes],  # type: ignore
                                  _class: Callable[[], Message] = ..., *,
                                  strict: Optional[bool] = ...) -> Message: ...
 

stdlib/3/email/message.pyi

@@ -11,7 +11,7 @@ from email.contentmanager import ContentManager
 
 _T = TypeVar('_T')
 
-_PayloadType = Union[List[Message], str]
+_PayloadType = Union[List[Message], str, bytes]
 _CharsetType = Union[Charset, str, None]
 _ParamsType = Union[str, None, Tuple[str, Optional[str], str]]
 _ParamType = Union[str, Tuple[Optional[str], Optional[str], str]]
@@ -25,7 +25,8 @@ class Message:
     def set_unixfrom(self, unixfrom: str) -> None: ...
     def get_unixfrom(self) -> Optional[str]: ...
     def attach(self, payload: 'Message') -> None: ...
-    def get_payload(self, i: int = ..., decode: bool = ...) -> _PayloadType: ...
+    def get_payload(self, i: int = ..., decode: bool = ...) \
+                    -> Optional[_PayloadType]: ...
     def set_payload(self, payload: _PayloadType,
                     charset: _CharsetType = ...) -> None: ...
     def set_charset(self, charset: _CharsetType) -> None: ...