Of course, code improvement are appreciated! However, it's not clear to me how much effort this requires on my part. Since I don't have much free time nowadays, I cannot promise I can respond in a timely manner.

I'll try not to require too much from you.
I'll do the interventions.
In case that I'll have questions, I'll be happy to consult.

After that, you'll ask you to review the PR.
Most interventions are simple and I'll document and justify them so I hope it will not take you too much time.

I fixed the pylint alert and created a PR.
Note that this is still work in progress yet I wanted to consult you.

I notice that the setup for development doe not use requirements file.
Is setting the development environment done using pyenv_setup.py? Can you guide me?

The alert in src\rinoh\hyphenator.py seems to identify a bug and not a refactoring opportunity.all = ("Hyphenator") while it should be a list (e.g. all = ["Hyphenator"]). https://stackoverflow.com/questions/44834/what-does-all-mean-in-python

In src\rinoh\highlight.py and src\rinoh\frontend\sphinx\nodes.py there are alerts on using-constant-test.
In both cases the function "warn" appears in the if statement but not as a call (e.g., warn()).
Later the function is called. What is the intention?
elif warn:
warn('Could not lex literal_block as "%s". '
'Highlighting skipped.' % language)

In src\rinoh\frontend\rst\nodes.py there is an alert on pointless statment.
Are properties being created on the fly and the self.title aimed to check the existence?
class Admonition(DocutilsGroupingNode):
grouped_flowables_class = rt.Admonition

def children_flowables(self, skip_first=0):
try:
self.title
return super().children_flowables(skip_first=1)
except AttributeError:
return super().children_flowables()

In src\rinoh\element.py method build_document of class DocumentElement has an empty implementation of pass.
It is implemented in subclasses. Might be better to enforce it by raising NotImplementedError.

In src\rinoh\stylesheets\matcher.py there is a wildcard import.
Code uses from rinoh.styleds import *
This hides the imported objects and might lead to collisions in the future if objects of the same name are imported with wildcards.  styleds is a collection of imports, aimed to ease related imports.
I copied the used imports from there and deleted the unused ones.  
This adds many imports and does not  benefit from styleds, so this is a downside of fixing this alert.

@brechtm, can you guide me regadding these cases?