[fix] issues reported by Pylint 4.0

Last major update of Pylint brings some breaking changes [1], fixed in this
patch.

[1] https://pylint.readthedocs.io/en/latest/whatsnew/4/4.0/index.html#breaking-changes

.pylintrc

@@ -162,7 +162,7 @@ no-docstring-rgx=^_
 property-classes=abc.abstractproperty
 
 # Regular expression matching correct variable names
-variable-rgx=(([a-z][a-zA-Z0-9_]{2,30})|(_[a-z0-9_]*)|([a-z]))$
+variable-rgx=([a-zA-Z0-9_]*)$
 
 
 [FORMAT]