fsmonitor: fix str/bytes mismatch when accessing watchman version
There were 2 bugs here. First, keys in the tuple are always
str. Second, we needed to normalize the value to bytes to
prevent a str/bytes mismatch on Python 3.
With this commit, `hg debuginstall` with fsmonitor enabled now
works on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D7205
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