Allow packages to be skipped by `rapids-upload-to-anaconda` (#32)
### Problem
Most of the Anaconda packages that our libraries build are intended to
be published to Anaconda.org.
However, `cugraphops` has some packages that they don't wish to
distribute.
Currently the `rapids-upload-to-anaconda` script indiscriminately
uploads all `conda` packages that aren't test packages (e.g.
`libcudf-tests`, `libcugraph-tests`, etc) to Anaconda.org
### Solution
This PR adds a new script, `rapids-find-anaconda-uploads.py`, that is
used by `rapids-upload-to-anaconda` to determine the list of packages
that should be uploaded to Anaconda.org.
`rapids-find-anaconda-uploads.py` will find all of the conda packages
within a specified directory and filter out any test packages and any
packages listed in the `SKIP_UPLOAD_PKGS` environment variable.
This PR also includes some `pytest`s for
`rapids-find-anaconda-uploads.py`.
### Additional Notes
These changes are a bit awkward due to the fact that they introduce a
Python script into an otherwise all-bash script repository.
However, I think this decision is warranted due to the complexity of the
logic in `rapids-find-anaconda-uploads.py`.
I initially tried implementing it in bash, but found that it was very
verbose and difficult to understand.
The ergonomics of Python enabled the resulting script to be super
succinct and has the added benefit of being unit-testable.
