Cellprofiler

Minor change to the watershed module for backwards compatibility.

Fixes bug preventing users to download .mat files from s3.

Support s3 URLs, via boto3. See [instructions for setup](https://github.com/CellProfiler/CellProfiler/wiki/s3).

Motivation: Users were unable to install CP2.2.1 correctly in pure pip.

Changes:
* New dependency pinning (per 3488) ensure CP2.2.2 can be installed in pure pip
* wx < 3 has been yanked, new installations will can only run in headless
* Numerous compatibility changes and backported fixes (per 3455 & chris-allan)