Pyrsl

- Updated RSL parser to support regex changes for Python >= 3.11 (related to https://github.com/xtuml/pyxtuml/issues/49)
- Updated project setup and deprecated Python 2 support (https://github.com/john-tornblom/pyrsl/issues/5)

- Update CI/CD to use GitHub actions

* Add ability to order query sets for select_many. [Example](http://pyrsl.readthedocs.io/en/master/language-reference.html#ordering-selections).
* Fix parsing of '.end if', '.end while', and '.end for' with trailing comments. [Bugreport](https://github.com/xtuml/pyrsl/issues/18).
* Fix runtime cardinality check of 'select one inst related by ...' statements

* fixes parsing issues with comments (see https://github.com/xtuml/pyrsl/issues/18)
* improves gtksourceview syntax highlighting
* installs gtksourceview syntax highlighting from setup.py (used by e.g. `pip install pyrsl`)

Documentation
-------------------
* reworked the language reference, see http://pyrsl.readthedocs.io

Setup
-------
* bump pyxtuml requirements to 2.0b1
* improve dependency installation when installing from pypi.org

Syntax
--------
* allow unique_id type parameters
* **allow specification of key letters within angle brackets for applicable parameter types.** [Example](http://pyrsl.readthedocs.io/en/master/language-reference.html#defining-functions)
* **add support for "relate/unrelate using" syntax, inspired by OAL syntax.** [Example](http://pyrsl.readthedocs.io/en/master/language-reference.html#connecting-instances)

Runtime
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* **reverse navigation phrases as used in pyxtuml v2**
* **make referential attributes read-only (as implied by the upgrade of pyxtuml to version 2)**
* check that parameter count == argument count in function invocations
* when a fragment is printed, display its key/values
* improve type checking of instance and instance set parameters