Pip-lock

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* Drop Python 3.5 support.
* Support Python 3.9.
* Move license from ISC to MIT License.

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* Fix to ignore packages installed directly from wheels, for example pip itself
on Heroku, which will output its location in ``pip freeze`` as
``pip file:///tmp/pip-20.1.1-py2.py3-none-any.whl``.

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* Update Python support to 3.5-3.8, as 3.4 has reached its end of life.
* Converted setuptools metadata to configuration file. This meant removing the
``__version__`` attribute from the package. If you want to inspect the
installed version, use
``importlib.metadata.version("pip-lock")``
(`docs <https://docs.python.org/3.8/library/importlib.metadata.html#distribution-versions>`__ /
`backport <https://pypi.org/project/importlib-metadata/>`__).
* Fix parsing of package names featuring extras e.g. ``package[extra1]``.
* Require Pip 10+.

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* Drop Python 2 support, only Python 3.4+ is supported now.

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* Ignore installed external (``-e``) packages.

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* Fix for pip 10 move of import to ``pip._internal``