Constrain cython=0.29

[mattwthompson]

Band-aid for #1800, which should be followed-up by a proper fix.

Version 3.0 (!) was released, but the current source code is only compatible with versions 0.29.x

[jeremyleung521]

I believe pip syntax requires a double equals sign == for version specification in pyproject.toml. test.yaml should be fine though since that's managed by conda.

[mattwthompson]

So it was passing with 0.29.36 a few days ago:
https://github.com/mdtraj/mdtraj/actions/runs/5570950610/jobs/10175514121#step:3:272

It's now failing with ... 0.29.36: https://github.com/mdtraj/mdtraj/actions/runs/5588508191/jobs/10215400973#step:3:272

Something else must have changed - I haven't looked deeper

[mattwthompson]

Oh, it's probably that I'm looking at the Cython in the conda environment, but a different one is used for the build process

[mattwthompson]

Thanks for the help @jeremyleung521 - this looks good to me, what do you think?

[jeremyleung521]

I suggest doing ==0.29.* or ~=0.29.36 just in case there is a newer 0.29.* bugfix somewhere down the line, but other than that, it looks good to me!

[mattwthompson]

Oh, nice - conda doesn't support the tilde specifier so I didn't think of that