Vcspull

- Fix pyproject.toml release version
- Fixes to contribution docs

- :issue:`257` - Move from Pipfile to poetry
- Removed dependency on releases for CHANGES, use plain reStructuredText
for now
- Loosen constraints for colorama
- Update click to 7/8
- Add `project_urls` to setup.py

- Sort imports
- Add isort package, isort configuration in setup.cfg, and
`make isort` task to Makefile.
- Add `make sync_pipfile` for updating _Pipfile_
dependencies against _requirements/\*.txt_ files.
- Update Pipfile
- Package updates

Base / Core dependencies:

- kaptan: 0.5.9 to >=0.5.9,<1 (adds Python 3.7 compatibility)
- pyyaml: >=3.12,<4 (adds Python 3.7 compatibility)

Docs:

- sphinx: 1.7.1 to 1.7.5
- alagitpull: 0.0.19 to 0.0.21

- Move to new organization, vcs-python
- Fix a bug in libvcs that prevented syncing on Ubuntu 18.04 LTS
beta.
- Update libvcs to 0.3.0
- Various package updates for support infrastructure,
including pytest and sphinx

- Update more documentation domain at vcspull.git-pull.com

- Update to new documentation domain at vcspull.git-pull.com
- Fix travis tests via `pip install -u mock` in
`.travis.yaml`.