CI: add conda-forge as lowest priority channel
xref #14495 

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14509?src=pr) is 85.27% (diff: 100%)

> Merging [#14509](https://codecov.io/gh/pandas-dev/pandas/pull/14509?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

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> Powered by [Codecov](https://codecov.io?src=pr). Last update [1d95179...a8e60c2](https://codecov.io/gh/pandas-dev/pandas/compare/1d951794187b72841d59b3dfde0a98309a64dec7...a8e60c2fa6716974e8ec493ab57fd766cf9bfeea?src=pr)
 @jreback The failing test for the python 3.4 build is due to a very old version of sqlalchemy that is installed (0.7.1). Not sure where this is coming from, as the requirements file do not specify a version.
 Ah, it's coming from the `pandas` conda channel. I suppose the conda-forge channel had priority over that before. 
Is there a reason you want conda-forge as the lowest priority?
 @jorisvandenbossche yeah maybe I should just take this out of the pandas channel. The idea with conda-forge is that we can pick up things that are _not_ in the default channel (quicker), but if they are already there, use them. The reasoning is that most people are using _defaults_, so we want to be exposed to that pipeline first.

The case here is `bottleneck==1.2.0` which is only on conda-forge (ATM).
 That sounds good as a reasoning. 
Are there actually still packages in the pandas channel we need? I suppose eg `psycopg2` is in the meantime also available in either defaults or conda-forge

Is specifying a version in a requirements file enough to trigger it to get it from conda-forge?

https://github.com/conda-forge/bottleneck-feedstock/pull/7 is merged in the meantime, so you can specify 1.2.0 for one of the builds?
 yeah I need to add the new version on a build (unfort have to manually specify it).

yes, can prob remove the need for the pandas channel entirely (if we have conda-forge), or maybe will add that one _after_. will try later
 closing for now. have better ways to do this esp when you need to specify more things than a channel, e.g. 34_SLOW build. / 3.6 build
