Correctly handle trailing whitespace on URL requirements

[pradyunsg]

Traceback (most recent call last):
  File "generate.py", line 38, in <module>
    req = Requirement(raw_req)
  File "/home/ichard26/programming/oss/black-deps-ci/venv/lib/python3.8/site-packages/packaging/requirements.py", line 37, in __init__
    raise InvalidRequirement(str(e)) from e
packaging.requirements.InvalidRequirement: Expected end or semicolon (after URL and whitespace)
    aiohttp @ https://github.com/aio-libs/aiohttp/archive/master.zip 
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

heh I did not expect packaging to become stricter on terminating whitespace

Reported by @ichard26 over Discord (if you're on the PyPA discord: https://discord.com/channels/803025117553754132/803025117995073578/1052081745446567986)

[ichard26]

Thanks!