Add fdscan() method to ClamdUnixSocket

[joelsmith]

This method is useful when clamd is running as a non-root user
and can't directly open a file that needs to be scanned. In
such cases, the clamd client can open the file and submit
a scan job via the open file descriptor.

[joelsmith]

@graingert Please take a look.

It looks like the py26 test is failing because no python 2.6 interpreter is available in the test environment. All the other tests are passing (except for pypy, which is skipping the new tests because it lacks the needed functionality in its python for the new feature).

Please let me know anything I should do differently.

Thanks!

[GreatBahram]

What is the problem with this pull request? @graingert

For future readers, the socket module has a send_fds (Python 3.9+) method, and multiprocessing.reduction.sendfds is available as well.

[graingert]

Please do drop stages for EOL pythons and support only 3.6,3.7,3.8,3.9 and 3.10

You'll probably need to add GitHub actions as Travis is EOL