Improve error shown when there's no space between semicolon and url

[Jackenmen]

Fixes #529

I kind of wanted to also show:

Expected space and semicolon (followed by markers) or end of string

when url doesn't end with ; but this sentence seemed a bit unnatural to me and I couldn't think of a better way of phrasing it + if the user ends up not putting a space, they'll get the better message when they try for the second time anyway.

I also originally wanted to say something more similar to Python's error messages, i.e. "Maybe you meant to put a space before the semicolon?" but that doesn't quite fit with the error message format ending with "at position ..":

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/work/packaging/packaging/requirements.py", line 42, in __init__
    raise InvalidRequirement(str(e))
packaging.requirements.InvalidRequirement: Expected semicolon (followed by markers) or end of string. Maybe you meant to put a space before the semicolon?
at position 59:
    tomli @ https://github.com/hukkin/tomli/archive/master.zip; python_version < '3.11'
                                                              ^

Either way, I'm open to suggestions.

[pradyunsg]

We could add a “hint: maybe you meant…” line to the error message.

[Jackenmen]

You mean like this?

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/work/packaging/packaging/requirements.py", line 42, in __init__
    raise InvalidRequirement(str(e))
packaging.requirements.InvalidRequirement: Expected semicolon (followed by markers) or end of string. Hint: maybe you meant to put a space before the semicolon?
at position 59:
    tomli @ https://github.com/hukkin/tomli/archive/master.zip; python_version < '3.11'
                                                               ^

Instead of the currently proposed:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/work/packaging/packaging/requirements.py", line 42, in __init__
    raise InvalidRequirement(str(e))
packaging.requirements.InvalidRequirement: Expected space before semicolon (followed by markers) or end of string
at position 58:
    tomli @ https://github.com/hukkin/tomli/archive/master.zip; python_version < '3.11'
                                                              ^

[uranusjr]

I think Pradyun means something like

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/work/packaging/packaging/requirements.py", line 42, in __init__
    raise InvalidRequirement(str(e))
packaging.requirements.InvalidRequirement: Expected space before semicolon (followed by markers) or end of string
at position 58:
    tomli @ https://github.com/hukkin/tomli/archive/master.zip; python_version < '3.11'

Maybe you mean this instead?
    tomli @ https://github.com/hukkin/tomli/archive/master.zip ; python_version < '3.11'