Port mantis backend to a maintained soap library

[dktrkranz]

mantis backend does use of the python-suds module, which is now deprecated due to long time missing upstream maintenance as well as missing compatibility for Python3.

Please consider porting code to use a maintained soap library (like pysimplesoap).

[nekohayo]

I'm working on putting this project back on its feet with the goal of getting a release out of the door.

This issue here is up for grabs for anyone wanting to see it happen. Please test with the development version, and contribute code (or bring in someone who can) to the project to bring this plugin back into good shape, or it risks getting decommissioned as part of issue #222.

[willismonroe]

I'm not able to test with a MantisBT instance, but it seems like it could be as simple as just swapping something like zeep in for python-suds in the import line, since zeep also has a "Client" class.

I noticed that Mantis now has a REST api and I wonder if it might make more sense to move over to that instead of SOAP.