try to use pip instead of conda-build for develop install

[munkm]

PR Summary

A number of recent PRs are failing on appveyor because conda-build can't be installed in the conda environment and needs to be installed in the root environment. It looks like we use conda-build only to be able to do a development build of yt with conda develop -b. Maybe the appveyor builds can be fixed by using pip instead.

[munkm]

The Python 2.7 builds fail on appveyor, but this is due to the updated numpy support that we added from #2448.

#2450 removes the python 2 testing from appveyor, so I think these failures shouldn't block merging this PR.

[neutrinoceros]

• I tried to look up pip's documentation for a replacement of the -b flag (build extensions in place) in conda develop -b ., but couldn't find one.
• Meanwhile, the build still fails on python 3 with a surprising (to say the least) error
  AttributeError: module 'numpy' has no attribute 'divmod'

Do you think there is a chance those two points are related ?

[neutrinoceros]

update: seems like pip doesn't have an equivalent flag by design and relies on the environment manager to invoke python setup.py build_ext --inplace (see pypa/pip#7509)

[munkm]

@neutrinoceros the divmod errors are because the numpy version that's being pulled by appveyor is too old (it's 1.11) to be compatible with our changes that added 1.18 support. I'm not sure why appveyor is pulling such old versions of numpy for both Python versions right now; when the 1.18 support PR was merged it was testing against numpy 1.16.

[munkm]

Ok, it seems that the changes introduced by conda have been remedied and this isn't an issue anymore (thanks @rebeccabilbro for figuring it out!). I retriggered the appveyor tests and we can install yt from source with pip install rather than conda develop. The failing tests for Python 3.6 are for matplotlib 3.2 (which I'm working on in another PR).

If anybody feels strongly about using pip install instead of conda develop in our appveyor scripts I'm happy to reopen this PR. However, I don't think this is a change that needs to be merged.