Add CI job targeting Traits 6.0

[kitchoi]

Closes #632

This PR adds a CI job to test against Traits 6.0.

Tests for the data views are skipped and associated imports requiring traits.api.observe are wrapped in a condition (in tests only).
Data view is a new feature not yet released so it is okay for it to depend on Traits 6.1 as downstream projects can opt-in.

[codecov-commenter]

Codecov Report

Merging #656 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #656      +/-   ##
==========================================
+ Coverage   39.23%   39.26%   +0.02%     
==========================================
  Files         491      491              
  Lines       26899    26910      +11     
  Branches     4065     4065              
==========================================
+ Hits        10555    10566      +11     
  Misses      15890    15890              
  Partials      454      454              

Impacted Files	Coverage Δ
pyface/__init__.py	82.60% <ø> (ø)
pyface/util/testing.py	89.47% <100.00%> (+4.28%)	⬆️

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[rahulporuri]

LGTM with two small questions.

[kitchoi]

Added "wx" to the job.

For the record, I did not add all the Qt backends because I think running just one of the Qt backends would achieve the same protection for the compatibility with Traits 6.0, compared to running all of the Qt backends. Then the choice of pyqt5 among pyqt and pyside2 is somewhat arbitrary.

[rahulporuri]

still LGTM