Convert mostly to python3 (still need strings/bytes conversions)

This covers conversion of the whole project to python3, *except* for
the strings/bytes distinction in __init__.py, which i'm leaving for a
second commit.

The changes in this commit are intended to be relatively
uncontroversial, so that the following commit contains the tricky
bits.

tests/dkimpy-milter

@@ -1,2 +1,2 @@
 #!/bin/sh
-python2 -m dkimpy_milter "$@"
+python3 -m dkimpy_milter "$@"