path library deprecated abspath() over 16.13.0

[xtrusia]

Checklist

• Confirmed this is an issue with charm-tools, not charmstore-client
• Provide versions of tools used
• Described the feature or ways to replicate the issue

Issue/Feature

charm build fails with
"AttributeError: 'Path' object has no attribute 'abspath'"

path library deprecated abspath() over 16.13.0

charm code also can handle this but it would be better if charm-tools can pin it under 16.13.0
https://github.com/jaraco/path/blob/e03580edf6cfec719890599010e0b164d06af50f/NEWS.rst#v16130

path is used here. and got error below
"AttributeError: 'Path' object has no attribute 'abspath'"

https://github.com/juju/charm-tools/blob/master/charmtools/build/builder.py#L881