Merge {IO,Environment,Windows}Error into OSError for Python 3.3+ (#1852)

Starting with Python 3.3, IOError, EnvironmentError and WindowsError are
aliases for OSError, which has all the attributes.

Reference:
* https://docs.python.org/3/library/exceptions.html#OSError
* https://www.python.org/dev/peps/pep-3151/

* OSError: Drop Python <3.3 compatibility
* Use Any instead of Union for filename/filename2 type, per GvR comment
See: https://github.com/python/mypy/pull/4541

stdlib/3/builtins.pyi

@@ -936,11 +936,18 @@ class SystemExit(BaseException):
     code = 0
 class Exception(BaseException): ...
 class ArithmeticError(Exception): ...
-class EnvironmentError(Exception):
+class OSError(Exception):
     errno = 0
     strerror = ...  # type: str
-    # TODO can this be bytes?
-    filename = ...  # type: str
+    # filename, filename2 are actually Union[str, bytes, None]
+    filename = ...  # type: Any
+    if sys.version_info >= (3, 4):
+        filename2 = ...  # type: Any
+    if sys.platform == 'win32':
+        winerror = 0
+IOError = OSError
+EnvironmentError = OSError
+WindowsError = OSError
 class LookupError(Exception): ...
 class RuntimeError(Exception): ...
 class ValueError(Exception): ...
@@ -949,7 +956,6 @@ class AttributeError(Exception): ...
 class BufferError(Exception): ...
 class EOFError(Exception): ...
 class FloatingPointError(ArithmeticError): ...
-class IOError(EnvironmentError): ...
 class ImportError(Exception):
     if sys.version_info >= (3, 3):
         name = ...  # type: str
@@ -961,7 +967,6 @@ class KeyError(LookupError): ...
 class MemoryError(Exception): ...
 class NameError(Exception): ...
 class NotImplementedError(RuntimeError): ...
-class OSError(EnvironmentError): ...
 class BlockingIOError(OSError):
     characters_written = 0
 class ChildProcessError(OSError): ...
@@ -978,8 +983,6 @@ class NotADirectoryError(OSError): ...
 class PermissionError(OSError): ...
 class ProcessLookupError(OSError): ...
 class TimeoutError(OSError): ...
-class WindowsError(OSError):
-    winerror = ...  # type: int
 class OverflowError(ArithmeticError): ...
 class ReferenceError(Exception): ...
 class StopIteration(Exception):