Add partial stubs for fractions (#544)

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`.

stdlib/2and3/fractions.pyi

@@ -0,0 +1,94 @@
+# Stubs for fractions
+# See https://docs.python.org/3/library/fractions.html
+#
+# Note: these stubs are incomplete. The more complex type
+# signatures are currently omitted. Also see numbers.pyi.
+
+from typing import Optional, TypeVar, Union, overload
+from numbers import Real, Integral, Rational
+from decimal import Decimal
+import sys
+
+_ComparableNum = Union[int, float, Decimal, Real]
+
+
+@overload
+def gcd(a: int, b: int) -> int: ...
+@overload
+def gcd(a: Integral, b: int) -> Integral: ...
+@overload
+def gcd(a: int, b: Integral) -> Integral: ...
+@overload
+def gcd(a: Integral, b: Integral) -> Integral: ...
+
+
+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): ...
+
+    def __hash__(self) -> int: ...
+    def __eq__(self, other: object) -> bool: ...
+    def __lt__(self, other: _ComparableNum) -> bool: ...
+    def __gt__(self, other: _ComparableNum) -> bool: ...
+    def __le__(self, other: _ComparableNum) -> bool: ...
+    def __ge__(self, other: _ComparableNum) -> 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': ...