gnu: pybitmessage: Update to 0.6.2.

* gnu/packages/messaging.scm (pybitmessage): Update to 0.6.2.
[build-system]: Change to python-build-system.
[arguments]: Remove "fix-makefile" and "wrap" phases.
[inputs]: Rename to...
[propagated-inputs]: ...this. Add "python2-msgpack" and "python2-pythondialog".
[inputs]: Remove "sqlite" and "qt-4". Move "openssl" ...
[native-inputs]: ...here. Remove "pkg-config".

Signed-off-by: Christopher Baines <mail@cbaines.net>