py3: fix bytes/unicode issues in convert/darcs
- don't check for a binary symbol in globals(), which meant it always thought
the module wasn't available
- don't pass bytes to stdlib methods
- return bytes in getchanges where Mercurial expects to see them
convert: remove old ElementTree import cruft from darcs
All the `import elementtree` attempts seem to pre-date py2.5, when it was
brought into the standard library, and the manual `cElementTree` fast
implementation import has been unnecessary and deprecated since py3.3.
Added signature for changeset
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