Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/html/__init__.pyi

@@ -0,0 +1,4 @@
+from typing import AnyStr
+
+def escape(s: AnyStr, quote: bool = ...) -> AnyStr: ...
+def unescape(s: AnyStr) -> AnyStr: ...

stdlib/@python2/html/entities.pyi

@@ -0,0 +1,6 @@
+from typing import Dict
+
+name2codepoint: Dict[str, int]
+html5: Dict[str, str]
+codepoint2name: Dict[int, str]
+entitydefs: Dict[str, str]

stdlib/@python2/html/parser.pyi

@@ -0,0 +1,30 @@
+from _markupbase import ParserBase
+from typing import List, Optional, Tuple
+
+class HTMLParser(ParserBase):
+    def __init__(self, *, convert_charrefs: bool = ...) -> None: ...
+    def feed(self, data: str) -> None: ...
+    def close(self) -> None: ...
+    def reset(self) -> None: ...
+    def getpos(self) -> Tuple[int, int]: ...
+    def get_starttag_text(self) -> Optional[str]: ...
+    def handle_starttag(self, tag: str, attrs: List[Tuple[str, Optional[str]]]) -> None: ...
+    def handle_endtag(self, tag: str) -> None: ...
+    def handle_startendtag(self, tag: str, attrs: List[Tuple[str, Optional[str]]]) -> None: ...
+    def handle_data(self, data: str) -> None: ...
+    def handle_entityref(self, name: str) -> None: ...
+    def handle_charref(self, name: str) -> None: ...
+    def handle_comment(self, data: str) -> None: ...
+    def handle_decl(self, decl: str) -> None: ...
+    def handle_pi(self, data: str) -> None: ...
+    def unknown_decl(self, data: str) -> None: ...
+    CDATA_CONTENT_ELEMENTS: Tuple[str, ...]
+    def check_for_whole_start_tag(self, i: int) -> int: ...  # undocumented
+    def clear_cdata_mode(self) -> None: ...  # undocumented
+    def goahead(self, end: bool) -> None: ...  # undocumented
+    def parse_bogus_comment(self, i: int, report: bool = ...) -> int: ...  # undocumented
+    def parse_endtag(self, i: int) -> int: ...  # undocumented
+    def parse_html_declaration(self, i: int) -> int: ...  # undocumented
+    def parse_pi(self, i: int) -> int: ...  # undocumented
+    def parse_starttag(self, i: int) -> int: ...  # undocumented
+    def set_cdata_mode(self, elem: str) -> None: ...  # undocumented