.pylintrc: Add ujson to extension-pkg-whitelist
Add ujson
to the extension-pkg-whitelist
(we trust it!). Without
this, pylint
throws the following (non-fatal) warning:
src/buildstream/_cachekey.py:65:14: I1101: Module 'ujson' has no 'dumps'
member, but source is unavailable. Consider adding this module to
extension-pkg-whitelist if you want to perform analysis based on
run-time introspection of living objects. (c-extension-no-member)