Automatic mergexp python tagging

This MR will make it so that with each new gitlab tag, a new version of mergexp on pypi will automatically be updated with that git tag. This removes the onus of always update the VERSION in setup.py for every change. Instead we now use versioneer to manage our python package versioning.

How it works:

Versioneer places some python files in the mergexp directory that manage the version. Now instead of a static version, the dynamic version of versioneer (which is a glorified git describe | cut | sed) manages the version in the _version.py which is based off setup.cfg. _version.py uses versioneer.py. No code updates need to occur to versioneer. This code will now manage versions without need to update our code.

This commit also adds some gitlab-ci to build the package, and on tags, push the package to pypi. The tags should be prefixed with 'v', if we change the prefix, we will need to modify setup.cfg with the new prefix and rerun python3 versioneer install which will regenerate the _version.py file. Versioneer does not need to be included in CI, but is included in requirements.txt to peg the version I used to create this, in case we need it later.