This pull request has changes to several files. The overall purpose is to get the entire MapScript Python test suite passing again, for both Python 2 and 3. There are now 293 tests, covering most of the MapScript API - up from 195 tests prior to this pull request.

Changes can be summarised as follows:

• Appveyor CI has been updated to build Python wheels and run the tests for both Python 2 and 3
• Fixing deprecation warnings for regex expressions in Python3
• Removing broken GD related imageObj creation code in Python MapScript and reverting to the generic functions to allow image creation
• Add a py2/3 fix for writing images, as several tests were failing due to writing unicode strings rather than bytes to files
• Moved the new dict hashTableObj to the pyextend.i file (where it should belong)
• Reintroduced previously failing test cases, and linted all Python test code (including MapScript thread and reference tests)
• Fixed various failing tests where old MapScript APIs were used (particularly with querying features)
• Fixed some whitespace issues in SWIG interface files causing warnings