CI: Inconsistencies in the CI tests
Not sure if anyone know exactly what we are testing, but after removing Python 3.5 and some other old stuff, I think we've got lots of builds testing almost the same things, and the lack of a clear structure is preventing us from finding inconsistencies easily.

Some things that seem wrong:
- I don't think we're running tests with `network` marker in any build (we run `not slow and not network` and `slow`)
- We just run the `slow` in allowed failures (not sure if that's intentional)
- `py36_locale_slow` is not actually running slow tests: https://github.com/pandas-dev/pandas/blob/master/ci/azure/posix.yml#L32
- We only test in Mac with 2 year old versions of numpy and dateutil (never with recent ones)
- In windows we also test only with old numpy versions 1.14 and 1.15, never recent ones

To fix those, what I think it'd make more sense is:
- Remove from the dependencies files the CI system (travis, azure) and the OS (windows...), I don't think those add value
- Get a dependencies file with all the dependencies without specifying versions (conda should give the latest) and test that on Linux, Windows and Mac
- Get a dependencies file with the minimum supported versions (similar to `azure-36-minimum_versions.yaml`, but including all dependencies, the current just got a small set). Test it in Linux (possibly in Windows and Mac if important, but I don't think so)
- Use the same dependencies files to test the slow and network tests
- For the locales, use couple of the previous builds
- For everything that we need to test that is not covered in the previous builds, create a dependencies file explaining exactly what we want to test with that (e.g. 32 bits, numpydev)

Not sure I'm missing something, but I think this should make things much simpler, test more things, and we can even save couple of builds (we have 12 now)

CC: @jreback @TomAugspurger 
