Issue 8750: Fixed MutableSet's methods to correctly handle reflexive operations, namely x -= x and x ^= x

fix issue 9601: ftplib now provides a workaround for invalid response code returned on MKD and PWD by non-compliant FTPserver implementations such as ISS shipped with Windows server 2003

os.path.dirname(__file__) points to the scrdir, not the builddir.
Use os.getcwd() instead.

to make test work on Solaris.
Disable gethostbyaddr IDNA test for now as there is no
reverse DNS entry for the IPv6 address of the test name.

test_importlib is that it discovers special little race conditions. For
instance, it turns out that importlib would throw an exception if two different
Python processes both tried to create the __pycache__ directory as one process
would succeed, causing the other process to fail as it didn't expect to get any
"help". So now importlib simply stays calm and just accepts someone else did
the work of creating the __pycache__ directory for it, moving on with life.

Closes issue #9572.

imports with an empty string in sys.path.

(part of #9657)

importlib._bootstrap, add the optional methods for importlib.abc.SourceLoader
for completeness.

variable the return value is assigned to is char *. Since the assigned-to
variable is never changed, simply make that a const char * and cast all calls
to get_history to const char * to silence the compiler warning (found with
LLVM).

getaddrinfo. Patch by David Watson.

(part of issue #9251)

ignored by the buffered IO layer anymore.

fix issue #9129: added proper error handling when accepting new connections in SMTPServer.handle_accept