DEPS: Unifying testing and building dependencies across builds
- [X] closes #29664

In our CI builds we're using different versions of pytest, pytest-xdist and other tools. Looks like Python 3.5 was forcing part of this, since some packages were not available.

In this PR I standardize the tools and versions we use in all packages, with only two exceptions:
- In 32 bits we install `Cython` from pip instead of conda, since conda doesn't have the version we want
- `pytest-azurepipelines` is installed only in builds currently running in pipelines

I also reorganize a bit the builds so dependencies are easier to find. First is always Python, then the block with the tools, which is always the same with the exceptions mentioned above, and finally the dependencies.