feat: use purl for pip requirements (#2067)

lib/manager/pip_requirements/extract.js

lib/manager/pip_requirements/index.js

lib/manager/pip_requirements/package.js

lib/manager/pip_requirements/update.js

lib/versioning/pep440/range.js

lib/workers/repository/process/lookup/index.js

test/_fixtures/pip_requirements/requirements1.txt

test/_fixtures/pip_requirements/requirements2.txt

...nager/pip_requirements/__snapshots__/extract.spec.js.snap

...nager/pip_requirements/__snapshots__/package.spec.js.snap

test/manager/pip_requirements/extract.spec.js

test/manager/pip_requirements/package.spec.js

test/manager/pip_requirements/update.spec.js