tests: normalize to bytes in test-install.t directory() was returning str and hgdirectory() was returning bytes. This made the set compare fail. Let's normalize the types on Python 3 so the test passes. Differential Revision: https://phab.mercurial-scm.org/D5650

config: reject str sections and keys on Python 3 Otherwise we could end up with a dict having both the str and bytes versions of a particular config item. This may cause some tests to regress. I haven't checked. But it is better behavior to fail fast. We could just as easily change this to normalize the values. But I like catching all non-compliant call sites first. Differential Revision: https://phab.mercurial-scm.org/D5649