Add Windows to the GitHub Actions CI test matrix

[kitchoi]

This is another incremental step towards moving what we currently have on Travis/Appveyor to GitHub Actions.

This PR adds Windows to the existing CI matrix.

One test being modified fails when we switch to the Windows worker from GitHub Actions. The test is trying to make sure that the ComponentEditor.width and ComponentEditor.height are taken into account for setting the component view size, but the upper bound in the test is only an approximation, because the window can be a bit larger due to borders and native screen size.

The test failure looks like this:

======================================================================
FAIL: test_initial_component_with_item_size (enable.tests.test_component_editor.TestComponentEditor)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\enable\enable\.edm\envs\enable-test-3.6-wx\lib\site-packages\enable\tests\test_component_editor.py", line 65, in test_initial_component_with_item_size
    self.assertLess(size[1], ITEM_HEIGHT+30)
AssertionError: 241 not less than 230

The 30 is a tolerance, instead of increasing the tolerance, I removed the assertion altogether.

[rahulporuri]

LGTM apart from a question/doubt.