package/python-decorator: bump to version 5.1.0

The pacakge doesn't support python2 anymore. Therefore, we can remove
host-python3-decorator and add a host variant for python-decorator.
As a consequence of that, the python-networkx dependency was renamed from
host-python3-decorator to host-python-decorator.

In addition, indent with two spaces in the hash file.

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>