Packageurl-python

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- Fix Python 2 compatibility issue 57

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- Add QuerySet utils to lookup and filter along the PackageURLMixin Django class 48
- Add a PackageURLFilter class for Django FilterSet implementations 48
- Move the django_models module to django.models 48
Replace `packageurl.contrib.django_models` imports with `packageurl.contrib.django.models`.

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- Document usage in README
- Adopt SPDX license identifier
- Add support for GitHub "raw" URLs in url2purl 43
- Improve GitHub support for "v" prefixed version in url2purl 43

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- Add and improve URL <-> Package URL conversion for gitlab, github, cargo,
bitbucket and hackage URL conversions
- Add new purl2url conversion utility
- Remove the null=True on Django CharField fields of the PackageURLMixin
- PackageURL.to_dict() now takes an optional "empty" argument with the value
that empty values to have. It defaults to None which was the current behaviour.
For some use cases, having an empty string may be a better option and this
enables this.

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- Make PackageURL hashable.
- Add cargo type or url2purl
- Increase the size of the Django model contrib version to 100 chars.
- Remove Python 3 idioms (f strings)

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- Add max length validation to the Django model contrib.