Avoid using string literals in type annotations (#2294)

stdlib/3/email/generator.pyi

@@ -5,7 +5,7 @@ from email.message import Message
 from email.policy import Policy
 
 class Generator:
-    def clone(self, fp: TextIO) -> 'Generator': ...
+    def clone(self, fp: TextIO) -> Generator: ...
     def write(self, s: str) -> None: ...
     def __init__(self, outfp: TextIO, mangle_from_: bool = ...,
                  maxheaderlen: int = ..., *,
@@ -14,7 +14,7 @@ class Generator:
                 linesep: Optional[str] =...) -> None: ...
 
 class BytesGenerator:
-    def clone(self, fp: TextIO) -> 'Generator': ...
+    def clone(self, fp: TextIO) -> Generator: ...
     def write(self, s: str) -> None: ...
     def __init__(self, outfp: TextIO, mangle_from_: bool = ...,
                  maxheaderlen: int = ..., *,

stdlib/3/email/headerregistry.pyi

@@ -12,7 +12,7 @@ class BaseHeader(str):
     def defects(self) -> Tuple[MessageDefect, ...]: ...
     @property
     def max_count(self) -> Optional[int]: ...
-    def __new__(cls, name: str, value: Any) -> 'BaseHeader': ...
+    def __new__(cls, name: str, value: Any) -> BaseHeader: ...
     def init(self, *args: Any, **kw: Any) -> None: ...
     def fold(self, *, policy: Policy) -> str: ...
 

stdlib/3/email/message.pyi

@@ -26,7 +26,7 @@ class Message:
     def is_multipart(self) -> bool: ...
     def set_unixfrom(self, unixfrom: str) -> None: ...
     def get_unixfrom(self) -> Optional[str]: ...
-    def attach(self, payload: 'Message') -> None: ...
+    def attach(self, payload: Message) -> None: ...
     def get_payload(self, i: int = ..., decode: bool = ...) -> Optional[_PayloadType]: ...
     def set_payload(self, payload: _PayloadType,
                     charset: _CharsetType = ...) -> None: ...
@@ -62,7 +62,7 @@ class Message:
     def set_boundary(self, boundary: str) -> None: ...
     def get_content_charset(self, failobj: _T = ...) -> Union[_T, str]: ...
     def get_charsets(self, failobj: _T = ...) -> Union[_T, List[str]]: ...
-    def walk(self) -> Generator['Message', None, None]: ...
+    def walk(self) -> Generator[Message, None, None]: ...
     if sys.version_info >= (3, 5):
         def get_content_disposition(self) -> Optional[str]: ...
     def as_string(self, unixfrom: bool = ..., maxheaderlen: int = ...,

stdlib/3/email/policy.pyi

@@ -16,7 +16,7 @@ class Policy:
     if sys.version_info >= (3, 5):
         mange_from = ...  # type: bool
     def __init__(self, **kw: Any) -> None: ...
-    def clone(self, **kw: Any) -> 'Policy': ...
+    def clone(self, **kw: Any) -> Policy: ...
     def handle_defect(self, obj: Message,
                       defect: MessageDefect) -> None: ...
     def register_defect(self, obj: Message,