Convert FileObjectClosend and cancel_wait_ex to classes

Instead of singleton instances. This is because exception objects are
stateful on Python 3, so these singletons could potentially be keeping
lots of extra state around.

greenlet.throw() accepts either an instance or a class so this results
in no extra internal changes.

CHANGES.rst

@@ -32,6 +32,11 @@
             under CPython and PyPy. gevent follows the CPython
             convention, although these functions cannot be called with
             keyword arguments on CPython.
+- The previously-singleton exception objects ``FileObjectClosed`` and
+  ``cancel_wait_ex`` were converted to classes. On Python 3, an
+  exception object is stateful, including references to its context
+  and possibly traceback, which could lead to objects remaining alive
+  longer than intended.
 
 1.2.1 (2017-01-12)
 ==================

src/gevent/_fileobjectcommon.py

@@ -6,8 +6,18 @@ except ImportError:
 
 from io import TextIOWrapper
 
-cancel_wait_ex = IOError(EBADF, 'File descriptor was closed in another greenlet')
-FileObjectClosed = IOError(EBADF, 'Bad file descriptor (FileObject was closed)')
+class cancel_wait_ex(IOError):
+
+    def __init__(self):
+        super(cancel_wait_ex, self).__init__(
+            EBADF, 'File descriptor was closed in another greenlet')
+
+
+class FileObjectClosed(IOError):
+
+    def __init__(self):
+        super(FileObjectClosed, self).__init__(
+            EBADF, 'Bad file descriptor (FileObject was closed)')
 
 class FileObjectBase(object):
     """

src/gevent/_socketcommon.py

@@ -204,7 +204,11 @@ def wait_readwrite(fileno, timeout=None, timeout_exc=_NONE, event=_NONE):
     return wait(io, timeout, timeout_exc)
 
 #: The exception raised by default on a call to :func:`cancel_wait`
-cancel_wait_ex = error(EBADF, 'File descriptor was closed in another greenlet') # pylint: disable=undefined-variable
+class cancel_wait_ex(error): # pylint: disable=undefined-variable
+    def __init__(self):
+        super(cancel_wait_ex, self).__init__(
+            EBADF,
+            'File descriptor was closed in another greenlet')
 
 
 def cancel_wait(watcher, error=cancel_wait_ex):

src/gevent/fileobject.py

@@ -175,7 +175,7 @@ class FileObjectThread(FileObjectBase):
                 # This is different than FileObjectPosix, etc,
                 # because we want to save the expensive trip through
                 # the threadpool.
-                raise FileObjectClosed
+                raise FileObjectClosed()
             with lock:
                 return threadpool.apply(method, args, kwargs)