Add partial stubs for fractions

This commit adds some incomplete stubs for the fractions module. In
particular, this commit does not add type signatures for the more
complex functions (such as `__add__`), and just leaves their types as
effectively `Any`.

Author: Michael0x2a

stdlib/2and3/fractions.pyi

@@ -0,0 +1,89 @@
+# Stubs for fractions
+# See https://docs.python.org/3/library/fractions.html
+
+from typing import Optional, TypeVar, Union, overload
+from numbers import Complex, Rational
+from decimal import Decimal
+import sys
+
+_TComplex = TypeVar('_TComplex', bound=Complex)
+_TOtherNum = TypeVar('_TOtherNum', int, float, Decimal)
+_TNum = TypeVar('_TNum', int, float, Decimal, Complex)
+
+@overload
+def gcd(a: _TComplex, b: _TComplex) -> _TComplex: ...
+@overload
+def gcd(a: _TOtherNum, b: _TOtherNum) -> _TOtherNum: ...
+
+class Fraction(Rational):
+    @overload
+    def __init__(self,
+                 numerator: Union[int, Rational] = 0,
+                 denominator: Optional[Union[int, Rational]] = 0,
+                 *,
+                 _normalize: bool = True) -> None: ...
+    @overload
+    def __init__(self, value: float, *, _normalize=True) -> None: ...
+    @overload
+    def __init__(self, value: Decimal, *, _normalize=True) -> None: ...
+    @overload
+    def __init__(self, value: str, *, _normalize=True) -> None: ...
+
+    @classmethod
+    def from_float(cls, f: float) -> 'Fraction': ...
+    @classmethod
+    def from_decimal(cls, dec: Decimal) -> 'Fraction': ...
+    def limit_denominator(self, max_denominator: int = 1000000) -> 'Fraction': ...
+
+    @property
+    def numerator(self) -> int: ...
+    @property
+    def denominator(self) -> int: ...
+
+    def __add__(self, other): ...
+    def __radd__(self, other): ...
+    def __sub__(self, other): ...
+    def __rsub__(self, other): ...
+    def __mul__(self, other): ...
+    def __rmul__(self, other): ...
+    def __truediv__(self, other): ...
+    def __rtruediv__(self, other): ...
+    if sys.version_info < (3, 0):
+        def __div__(self, other): ...
+        def __rdiv__(self, other): ...
+    def __floordiv__(self, other) -> int: ...
+    def __rfloordiv__(self, other) -> int: ...
+    def __mod__(self, other): ...
+    def __rmod__(self, other): ...
+    def __pow__(self, other): ...
+    def __rpow__(self, other): ...
+
+    def __pos__(self) -> 'Fraction': ...
+    def __neg__(self) -> 'Fraction': ...
+    def __abs__(self) -> 'Fraction': ...
+    def __trunc__(self) -> int: ...
+    if sys.version_info >= (3, 0):
+        def __floor__(self) -> int: ...
+        def __ceil__(self) -> int: ...
+        def __round__(self, ndigits=None): ...
+
+    # TODO: Once https://github.com/python/typeshed/pull/543 is
+    # accepted, add the following definition
+    # def __hash__(self) -> int: ...
+    def __eq__(self, other: object) -> bool: ...
+    def __lt__(self, other: _TNum) -> bool: ...
+    def __gt__(self, other: _TNum) -> bool: ...
+    def __le__(self, other: _TNum) -> bool: ...
+    def __ge__(self, other: _TNum) -> bool: ...
+    if sys.version_info >= (3, 0):
+        def __bool__(self) -> bool: ...
+    else:
+        def __nonzero__(self) -> bool: ...
+
+    # Not actually defined within fractions.py, but provides more useful
+    # overrides
+    @property
+    def real(self) -> 'Fraction': ...
+    @property
+    def imag(self) -> 'Fraction': ...
+    def conjugate(self) -> 'Fraction': ...