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

hgext/fsmonitor/__init__.py

@@ -190,7 +190,7 b' def debuginstall(ui, fm):'
         fm.write(
             b"fsmonitor-watchman-version",
             _(b" watchman binary version %s\n"),
-            v[b"version"],
+            pycompat.bytestr(v["version"]),
         )
     except watchmanclient.Unavailable as e:
         err = str(e)