gnu: python: Prefix expat license by "license:" to avoid conflicts with
expat from the xml module.
* gnu/packages/python.scm: Prefix expat license imported from
(guix licenses).
* gnu/packages/python.scm (python-lockfile, python-mock, python-pysam,
python-extras, python-mimeparse, python-py, python-pytest,
python-scripttest, python-jsonschema, python-pyjwt, python-virtualenv,
python-sphinx-rtd-theme, python-pyparsing, python-cffi, python-xcffib,
python-html5lib, python-dbus): Adapt license field.
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