Add stub for pyrfc3339 (#4564)

third_party/3/pyrfc3339/__init__.pyi

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+from .generator import generate as generate
+from .parser import parse as parse

third_party/3/pyrfc3339/generator.pyi

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+from datetime import datetime
+
+def generate(dt: datetime, utc: bool = ..., accept_naive: bool = ..., microseconds: bool = ...) -> str: ...

third_party/3/pyrfc3339/parser.pyi

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+from datetime import datetime
+
+def parse(timestamp: str, utc: bool = ..., produce_naive: bool = ...) -> datetime: ...

third_party/3/pyrfc3339/utils.pyi

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+from datetime import datetime, timedelta, tzinfo
+from typing import Optional
+
+class FixedOffset(tzinfo):
+    def __init__(self, hours: float, minutes: float) -> None: ...
+    def dst(self, dt: Optional[datetime]) -> timedelta: ...
+    def utcoffset(self, dt: Optional[datetime]) -> timedelta: ...
+    def tzname(self, dt: Optional[datetime]) -> str: ...
+
+def timedelta_seconds(td: timedelta) -> int: ...
+def timezone(utcoffset: float) -> str: ...