Str representation of Requirement is missing white space when url is present

[SaltumDis]

According to guide: https://packaging.python.org/en/latest/specifications/version-specifiers/#direct-references
Url representation should be like so:

pip @ file:///localbuilds/pip-1.3.1.zip

Currently white space before @ is missing

>>> from packaging.requirements import Requirement 
>>> str(Requirement("pip @ file:///localbuilds/pip-1.3.1.zip"))
'pip@ file:///localbuilds/pip-1.3.1.zip'

[miketheman]

I think the source of this is here:

packaging/src/packaging/requirements.py

Line 60 in 25b1f44

yield f"@ {self.url}"