Insist on Cython prior to 3.0

[drbsg]

This is a back-port of pull request #1061 to the 5.3 maintenance branch.

[jwiggins]

LGTM

@corranwebster Any objections? (you're a bit closer to this than me)

[corranwebster]

PR looks good, not sure what's happening with CI

[jwiggins]

Looks like the Wx environment needs attrdict installed before installing Wx

[drbsg]

Looks like the Wx environment needs attrdict installed before installing Wx

I am not familiar with EDM, so would adding this dependency to dependencies for both versions in ci/edmtool.py be a good way to achieve this?

[corranwebster]

I think the right way to fix would either be to restrict the WxPython version or to add attrdict here:

enable/enable/__init__.py

Line 52 in 84b611d

"wx": ["pyface[wx]", "traitsui[wx]"],

The problem seems to be this version of WxPython not declaring all dependencies.