Refine stubs for numbers.pyi (#543)

* Partially refine numbers.pyi

This commit refines numbers.pyi. More specifically, it...

- Adds in explicit type annotations where it's obvious how to do
  so (leaving more non-obvious type signatures alone).
- Adds in missing '@AbstractMethod' decorators
- Combines together the Python 2 and Python 3 versions of numbers.pyi.

* Merges Python 2 and Python 3 numbers.pyi

This commit removes the old `stdlib/2.7/numbers.pyi` file and moves the
previously-committed `stdlib/3/numbers.pyi` to the shared `2and3`
directory.

* Modify comment at top of numbers.pyi

Since the stubs are no longer identical to what stubgen creates, the
comment should also probably be changed.

Author: Michael0x2a

stdlib/2.7/numbers.pyi

@@ -0,0 +1,140 @@
+# Stubs for numbers (Python 3.5)
+# See https://docs.python.org/2.7/library/numbers.html
+# and https://docs.python.org/3/library/numbers.html
+#
+# Note: these stubs are incomplete. The more complex type
+# signatures are currently omitted.
+
+from typing import Any, Optional, TypeVar
+from abc import ABCMeta, abstractmethod
+import sys
+
+class Number(metaclass=ABCMeta):
+    @abstractmethod
+    def __hash__(self) -> int: ...
+
+class Complex(Number):
+    @abstractmethod
+    def __complex__(self) -> complex: ...
+    if sys.version_info >= (3, 0):
+        def __bool__(self) -> bool: ...
+    else:
+        def __nonzero__(self) -> bool: ...
+    @property
+    @abstractmethod
+    def real(self): ...
+    @property
+    @abstractmethod
+    def imag(self): ...
+    @abstractmethod
+    def __add__(self, other): ...
+    @abstractmethod
+    def __radd__(self, other): ...
+    @abstractmethod
+    def __neg__(self): ...
+    @abstractmethod
+    def __pos__(self): ...
+    def __sub__(self, other): ...
+    def __rsub__(self, other): ...
+    @abstractmethod
+    def __mul__(self, other): ...
+    @abstractmethod
+    def __rmul__(self, other): ...
+    if sys.version_info < (3, 0):
+        @abstractmethod
+        def __div__(self, other): ...
+        @abstractmethod
+        def __rdiv__(self, other): ...
+    @abstractmethod
+    def __truediv__(self, other): ...
+    @abstractmethod
+    def __rtruediv__(self, other): ...
+    @abstractmethod
+    def __pow__(self, exponent): ...
+    @abstractmethod
+    def __rpow__(self, base): ...
+    def __abs__(self): ...
+    def conjugate(self): ...
+    def __eq__(self, other: object) -> bool: ...
+    if sys.version_info < (3, 0):
+        def __ne__(self, other: object) -> bool: ...
+
+class Real(Complex):
+    @abstractmethod
+    def __float__(self) -> float: ...
+    @abstractmethod
+    def __trunc__(self) -> int: ...
+    if sys.version_info >= (3, 0):
+        @abstractmethod
+        def __floor__(self) -> int: ...
+        @abstractmethod
+        def __ceil__(self) -> int: ...
+        @abstractmethod
+        def __round__(self, ndigits: Optional[int] = None): ...
+    def __divmod__(self, other): ...
+    def __rdivmod__(self, other): ...
+    @abstractmethod
+    def __floordiv__(self, other): ...
+    @abstractmethod
+    def __rfloordiv__(self, other): ...
+    @abstractmethod
+    def __mod__(self, other): ...
+    @abstractmethod
+    def __rmod__(self, other): ...
+    @abstractmethod
+    def __lt__(self, other) -> bool: ...
+    @abstractmethod
+    def __le__(self, other) -> bool: ...
+    def __complex__(self) -> complex: ...
+    @property
+    def real(self): ...
+    @property
+    def imag(self): ...
+    def conjugate(self): ...
+
+class Rational(Real):
+    @property
+    @abstractmethod
+    def numerator(self) -> int: ...
+    @property
+    @abstractmethod
+    def denominator(self) -> int: ...
+    def __float__(self) -> float: ...
+
+class Integral(Rational):
+    if sys.version_info >= (3, 0):
+        @abstractmethod
+        def __int__(self) -> int: ...
+    else:
+        @abstractmethod
+        def __long__(self) -> long: ...
+    def __index__(self) -> int: ...
+    @abstractmethod
+    def __pow__(self, exponent, modulus=None): ...
+    @abstractmethod
+    def __lshift__(self, other): ...
+    @abstractmethod
+    def __rlshift__(self, other): ...
+    @abstractmethod
+    def __rshift__(self, other): ...
+    @abstractmethod
+    def __rrshift__(self, other): ...
+    @abstractmethod
+    def __and__(self, other): ...
+    @abstractmethod
+    def __rand__(self, other): ...
+    @abstractmethod
+    def __xor__(self, other): ...
+    @abstractmethod
+    def __rxor__(self, other): ...
+    @abstractmethod
+    def __or__(self, other): ...
+    @abstractmethod
+    def __ror__(self, other): ...
+    @abstractmethod
+    def __invert__(self): ...
+    def __float__(self) -> float: ...
+    @property
+    def numerator(self) -> int: ...
+    @property
+    def denominator(self) -> int: ...