Make 4.9.2 release containing fix for tests that depend on Categories

[mdickinson]

The test suite for Envisage 4.9.1 fails against the latest release of Traits (Traits 6.0.0), due to the removal of Traits categories. (Envisage master no longer depends on Traits categories; so this is only an issue in this release series.)

This PR "fixes" the issue by skipping the relevant tests if Traits categories are not available.

Also bumps the version number and updates the changelog in preparation for a 4.9.2 release.

[codecov-io]

Codecov Report

Merging #263 into release/4.9.x will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           release/4.9.x     #263   +/-   ##
==============================================
  Coverage          54.18%   54.18%           
==============================================
  Files                138      138           
  Lines               4503     4503           
  Branches             526      526           
==============================================
  Hits                2440     2440           
  Misses              2018     2018           
  Partials              45       45

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[corranwebster]

This is fine as is.

There may be a point to detecting that categories are unavailable and warning in the main envisage codebase (eg. if you try to instantiate an envisage.category.Category) if someone tries to use them, but that may be more effort than is worth it for this.

[mdickinson]

but that may be more effort than is worth it for this.

That was pretty much the conclusion I came to, given that all this is gone in the upcoming release.