Plac

Removed plac server based functionality which were asyncore based and as such deprecated in Python 3.10.

Ensured tests using plac_runner use the same version of Python as them.
Fixed the tests on Python 3.9 and 3.10 and enabled Travis for them.
Prevent the tests from cluttering the test user's home directory.
Removed the pre-generated documentation, please use Read the Docs.
Cleaned up many minor issues in the documentation.
Removed use of obsolete SQLSoup library and features that used it.
Fixed issue with str as type hint.
Added more tests/examples.

Thanks to Istvan Albert, it is now possible to use language keywords and
builtins as option/flag names. Some broken links were fixed and the
documentation has been moved to https://plac.readthedocs.io,
while the CI framework has changed from Travis to GitHub actions.

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.

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.

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.