tools: Fix generate-docs-nm-settings-docs-gir.py on Python 2

This a follow up to !1246 (closed)

@thaller found out that build was failing on EPEL7.

On python2 the following error is raised:

LookupError: unknown encoding: unicode

Seems like unicode is a correct encoding in Python 3 but not 2.

Fix:

1. Change encoding to utf-8
2. Pass output path string instead of opening file and passing opened file object. Python2 and 3 might need different file modes, passing just path lets ElementTree select appropriate file mode.

I tested element tree on python2 and it utf-8 seems to work. (I can't test entire build as it would require me to install too many Python 2 dependencies)