CI: add osx to travis, xref #8630
CI: remove numpy_dev 2.7 build
CI: remove extraneous requirements

@jorisvandenbossche note that this fixed the problem of having `statsmodels` removed after the build (and `xarray` as well), which was a problem in the doc-build
 [this](https://github.com/jreback/pandas/commit/b355c774f503edcf6c11d5584e84f18e40cd43b9) fixed the cython issues (wasn't building) in enhancedperf.rst.

note that technically we have a minimum requirement on ipython/cython for doc-building (e.g. whenever `Cython` replaced `cythongmagic`)
