Fix appveyor errors

[seadowg]

The version of formencode we're using seems to be having problems with appveyor. This fix is stolen from https://github.com/XLSForm/pyxform/pull/548.

Why is this the best possible solution? Were any other approaches considered?

From looking at the release notes for formencode 2.0.0 (here) it looks like it no longer works with Python 2 (or anything under Python 3.6). As far as I understand, pyxform still supports these versions, so I'm not sure if this upgrade actually is the right fix. An alternative might be to downgrade to 1.3.0 if that also works.

It could also be that there is an entirely different solution! I'm still not quite sure why appveyor is running into problems with formencode 1.3.1.

What are the regression risks?

See above.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Maybe if we're planning to drop support for some Python versions (although just to the README)

Before submitting this PR, please make sure you have:

• [x] included test cases for core behavior and edge cases in tests_v1
• [x] run nosetests and verified all tests pass
• [x] run black pyxform to format code
• [x] verified that any code or assets from external sources are properly credited in comments

[yanokwa]

Closing in favor of https://github.com/XLSForm/pyxform/pull/548.