Use Miniconda3

[humitos]

Update to use Miniconda3 considering that Python2 is about to die in a
couple of months.

The only change from a user perspective is that will make people not
defining the Python version in their environment.yml file to default
on Python 3.

Closes #86

[agjohnson]

Forcing the version of Python we call was a little easier, because we had control of the config file. Because we don't have direct control over the environment.yml file, this change seems like it would pretty immediately break builds. Is there a softer way of doing this?

[humitos]

@agjohnson I think using Miniconda3 is the right approach, so we stay tunned with the community.

My idea to not break old projects not setting the - python== version on their environment.yaml is to add a timed feature flag and append - python==2.7 to it. Using the same/similar code than in readthedocs/readthedocs.org#5956

[humitos]

Here, https://github.com/readthedocs/readthedocs.org/pull/5956/files#diff-31f8391d8643c47c6caedf53930c0b7eR530, _get_core_requirements could also include python==2.7 if the flag is enabled and everything will keep working.

[humitos]

It seems that Python2 will be supported forever. So, I'm closing this.