Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/email/policy.pyi

@@ -0,0 +1,54 @@
+from abc import abstractmethod
+from email.contentmanager import ContentManager
+from email.errors import MessageDefect
+from email.header import Header
+from email.message import Message
+from typing import Any, Callable, List, Optional, Tuple, Union
+
+class Policy:
+    max_line_length: Optional[int]
+    linesep: str
+    cte_type: str
+    raise_on_defect: bool
+    mange_from: bool
+    def __init__(self, **kw: Any) -> None: ...
+    def clone(self, **kw: Any) -> Policy: ...
+    def handle_defect(self, obj: Message, defect: MessageDefect) -> None: ...
+    def register_defect(self, obj: Message, defect: MessageDefect) -> None: ...
+    def header_max_count(self, name: str) -> Optional[int]: ...
+    @abstractmethod
+    def header_source_parse(self, sourcelines: List[str]) -> Tuple[str, str]: ...
+    @abstractmethod
+    def header_store_parse(self, name: str, value: str) -> Tuple[str, str]: ...
+    @abstractmethod
+    def header_fetch_parse(self, name: str, value: str) -> str: ...
+    @abstractmethod
+    def fold(self, name: str, value: str) -> str: ...
+    @abstractmethod
+    def fold_binary(self, name: str, value: str) -> bytes: ...
+
+class Compat32(Policy):
+    def header_source_parse(self, sourcelines: List[str]) -> Tuple[str, str]: ...
+    def header_store_parse(self, name: str, value: str) -> Tuple[str, str]: ...
+    def header_fetch_parse(self, name: str, value: str) -> Union[str, Header]: ...  # type: ignore
+    def fold(self, name: str, value: str) -> str: ...
+    def fold_binary(self, name: str, value: str) -> bytes: ...
+
+compat32: Compat32
+
+class EmailPolicy(Policy):
+    utf8: bool
+    refold_source: str
+    header_factory: Callable[[str, str], str]
+    content_manager: ContentManager
+    def header_source_parse(self, sourcelines: List[str]) -> Tuple[str, str]: ...
+    def header_store_parse(self, name: str, value: str) -> Tuple[str, str]: ...
+    def header_fetch_parse(self, name: str, value: str) -> str: ...
+    def fold(self, name: str, value: str) -> str: ...
+    def fold_binary(self, name: str, value: str) -> bytes: ...
+
+default: EmailPolicy
+SMTP: EmailPolicy
+SMTPUTF8: EmailPolicy
+HTTP: EmailPolicy
+strict: EmailPolicy