Pip-lock

------------------

* 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``.

------------------

* 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+.

------------------

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

------------------

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

------------------

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

------------------

* Remove logic that made relative file paths relative to the path of the
calling code's file. It's now the standard behaviour of relative to the
current working directory. Passing an absolute path is recommended.
* Make comparison of package names case-insensitive to work with
``requirements.txt`` files that use a different case to the canoncial package
name. This can happen with ``pip-compile`` that always outputs lowercase
names.
* Fix 'mismatches' typo
* Only indent mismatch list by 4 spaces in error message