[release] compatibility and bug fixes, cli improvements and code formatting

See merge request !39

Update psyplot min version

See merge request !38

implement CI matrix

See merge request !37

implement decoder command-line argument

See merge request !36

fix updating current projects in console

See merge request !35

migrate to psyplot-plugin-template

See merge request !34

As with psyplot 1.4.0, psyplot-gui is now continuously tested and deployed with CircleCI.

Added
-----
-   psyplot-gui does now have a CITATION.cff file, see https://citation-file-format.github.io

Changed
-------
-   psyplot is now officially licensed under LGPL-3.0-only, see [#29](https://github.com/psyplot/psyplot/pull/29)
-   Documentation is now hosted with Github Pages at https://psyplot.github.io/psyplot-gui. Redirects from the old documentation at [https://psyplot-gui.readthedocs.io]{.title-ref} have been configured.
-   We use CicleCI now for a standardized CI/CD pipeline to build and test the code and docs all at one place, see [#28](https://github.com/psyplot/psyplot-gui/pull/28)

* replace links to RTD

* fix license and copyright

* change license to LGPL

* include license change in changelog

* remove misleading requirements file

* add circleci project setup

* fix docs

* update appveyor config

* remove RTD config

* add environment file for ci

* remove redundant files

* update link to documentation

* install psutil for tests

* update changelog with circleci changes

use versioneer for version control

Disable intersphinx on windows by default

Revert jupyter_client restriction

i.e. c8e50da3