Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/string.pyi

@@ -0,0 +1,30 @@
+from typing import Any, Iterable, Mapping, Optional, Sequence, Tuple, Union
+
+ascii_letters: str
+ascii_lowercase: str
+ascii_uppercase: str
+digits: str
+hexdigits: str
+octdigits: str
+punctuation: str
+printable: str
+whitespace: str
+
+def capwords(s: str, sep: Optional[str] = ...) -> str: ...
+
+class Template:
+    template: str
+    def __init__(self, template: str) -> None: ...
+    def substitute(self, __mapping: Mapping[str, object] = ..., **kwds: object) -> str: ...
+    def safe_substitute(self, __mapping: Mapping[str, object] = ..., **kwds: object) -> str: ...
+
+# TODO(MichalPokorny): This is probably badly and/or loosely typed.
+class Formatter:
+    def format(self, __format_string: str, *args: Any, **kwargs: Any) -> str: ...
+    def vformat(self, format_string: str, args: Sequence[Any], kwargs: Mapping[str, Any]) -> str: ...
+    def parse(self, format_string: str) -> Iterable[Tuple[str, Optional[str], Optional[str], Optional[str]]]: ...
+    def get_field(self, field_name: str, args: Sequence[Any], kwargs: Mapping[str, Any]) -> Any: ...
+    def get_value(self, key: Union[int, str], args: Sequence[Any], kwargs: Mapping[str, Any]) -> Any: ...
+    def check_unused_args(self, used_args: Sequence[Union[int, str]], args: Sequence[Any], kwargs: Mapping[str, Any]) -> None: ...
+    def format_field(self, value: Any, format_spec: str) -> Any: ...
+    def convert_field(self, value: Any, conversion: str) -> Any: ...