Update plac to 1.2.0

[pyup-bot]

This PR updates plac from 1.0.0 to 1.2.0.

Changelog

1.2.0

Added dedenting of usage docstrings, as requested by Istvan Albert.
Added new decorators `plac.pos`, `plac.opt`, `plac.flg` and an example
using them in a section "For the impatient".
Added tests on travis for Python 3.8.

1.1.3

Fixed some issues with kwargs parsing, docstring formatting and empty
string defaults reported by the user https://github.com/isaacto. Changed
the testing framework on travis from nosetest to pytest. Ported the
documentation to sphinx.

1.1.0

Extended the recognition of default types to date and datetime in ISO
format. Fixed a bug when running plac scripts from Jupyter notebooks,
signaled by https://github.com/ursachi and https://github.com/rkpatel33.
Moreover, at user request, removed a Python 3.7 deprecation warning,
added a LICENSE.txt file and a Quickstart section to the README. plac
is tested on Travis for Python 2.7 and 3.4+ but it should work also
for all the other 3.X releases.

Links

• PyPI: https://pypi.org/project/plac
• Changelog: https://pyup.io/changelogs/plac/
• Repo: https://github.com/micheles/plac