Specify version for pygls

[kinow]

Installing as per docs, with pip install -r requirements.txt results in the following error:

(venv) kinow@ranma:~/Development/python/workspace/cwl-language-server$ pip install -r requirements.txt 
Collecting git+https://github.com/openlawlibrary/pygls.git@ (from -r requirements.txt (line 2))
ERROR: The URL 'git+https://github.com/openlawlibrary/pygls.git@' has an empty revision (after @) which is not supported. Include a revision after @ or remove @ from the URL.
WARNING: You are using pip version 22.0.4; however, version 22.1.2 is available.
You should consider upgrading via the '/home/kinow/Development/python/workspace/cwl-language-server/venv/bin/python -m pip install --upgrade pip' command.

I used the latest tag master (since the README file mentions it) from their GH repository to fix it.