[pkg] Configure "universal" wheel build
According to https://packaging.python.org/distributing/#wheels,
we should create universal wheels (no 2to3 step, no C extension).
Configure this in setup.cfg.
[ci skip]
[rqlrewrite] Test and fix potential NameError
We were referencing a loop variable, which may lead to a name error and show a
potential error if there are several matching variables.
To avoid this, introduce a list to hold every encountered variable and process
all of them later.