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/csv.pyi

@@ -0,0 +1,110 @@
+import sys
+from _csv import (
+    QUOTE_ALL as QUOTE_ALL,
+    QUOTE_MINIMAL as QUOTE_MINIMAL,
+    QUOTE_NONE as QUOTE_NONE,
+    QUOTE_NONNUMERIC as QUOTE_NONNUMERIC,
+    Dialect as Dialect,
+    Error as Error,
+    _DialectLike,
+    _reader,
+    _writer,
+    field_size_limit as field_size_limit,
+    get_dialect as get_dialect,
+    list_dialects as list_dialects,
+    reader as reader,
+    register_dialect as register_dialect,
+    unregister_dialect as unregister_dialect,
+    writer as writer,
+)
+from typing import Any, Generic, Iterable, Iterator, List, Mapping, Optional, Sequence, Text, Type, TypeVar, overload
+
+if sys.version_info >= (3, 8) or sys.version_info < (3, 6):
+    from typing import Dict as _DictReadMapping
+else:
+    from collections import OrderedDict as _DictReadMapping
+
+_T = TypeVar("_T")
+
+class excel(Dialect):
+    delimiter: str
+    quotechar: str
+    doublequote: bool
+    skipinitialspace: bool
+    lineterminator: str
+    quoting: int
+
+class excel_tab(excel):
+    delimiter: str
+
+if sys.version_info >= (3,):
+    class unix_dialect(Dialect):
+        delimiter: str
+        quotechar: str
+        doublequote: bool
+        skipinitialspace: bool
+        lineterminator: str
+        quoting: int
+
+class DictReader(Generic[_T], Iterator[_DictReadMapping[_T, str]]):
+    fieldnames: Optional[Sequence[_T]]
+    restkey: Optional[str]
+    restval: Optional[str]
+    reader: _reader
+    dialect: _DialectLike
+    line_num: int
+    @overload
+    def __init__(
+        self,
+        f: Iterable[Text],
+        fieldnames: Sequence[_T],
+        restkey: Optional[str] = ...,
+        restval: Optional[str] = ...,
+        dialect: _DialectLike = ...,
+        *args: Any,
+        **kwds: Any,
+    ) -> None: ...
+    @overload
+    def __init__(
+        self: DictReader[str],
+        f: Iterable[Text],
+        fieldnames: Optional[Sequence[str]] = ...,
+        restkey: Optional[str] = ...,
+        restval: Optional[str] = ...,
+        dialect: _DialectLike = ...,
+        *args: Any,
+        **kwds: Any,
+    ) -> None: ...
+    def __iter__(self) -> DictReader[_T]: ...
+    if sys.version_info >= (3,):
+        def __next__(self) -> _DictReadMapping[_T, str]: ...
+    else:
+        def next(self) -> _DictReadMapping[_T, str]: ...
+
+class DictWriter(Generic[_T]):
+    fieldnames: Sequence[_T]
+    restval: Optional[Any]
+    extrasaction: str
+    writer: _writer
+    def __init__(
+        self,
+        f: Any,
+        fieldnames: Sequence[_T],
+        restval: Optional[Any] = ...,
+        extrasaction: str = ...,
+        dialect: _DialectLike = ...,
+        *args: Any,
+        **kwds: Any,
+    ) -> None: ...
+    if sys.version_info >= (3, 8):
+        def writeheader(self) -> Any: ...
+    else:
+        def writeheader(self) -> None: ...
+    def writerow(self, rowdict: Mapping[_T, Any]) -> Any: ...
+    def writerows(self, rowdicts: Iterable[Mapping[_T, Any]]) -> None: ...
+
+class Sniffer(object):
+    preferred: List[str]
+    def __init__(self) -> None: ...
+    def sniff(self, sample: str, delimiters: Optional[str] = ...) -> Type[Dialect]: ...
+    def has_header(self, sample: str) -> bool: ...