set dependecy of pyyaml to version 5.4

[sabinem]

[wardi]

strict dependencies in install_requires causes cli commands to fail before they start if the version is ever changed. What is this change for? Can you use a version range instead?

[ccancellieri]

can be related to #305

[smotornyuk]

I may be wrong, but it shouldn't be possible. pip will not try to install a library if it has a minimum required python version specified. i.e, if pyyaml > 5.4 has python_requires > 3.0 flag, pip will never try to install it on python2. If this flag is missing, it's an issue of the dependency(pyyaml in this case) and must be solved in corresponding repository