Dependency specifiers grammar fixes

[konstin]

This PR contains three changes:

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Dependency Specifiers: Don't require whitespace after URL

otherwise numpy @ https://example.org/numpy is not a valid specifier (missing whitespace after the URL). This already what pip and packaging do and also what the parsley grammar says

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Dependency Specifiers: Require whitespace before in and not in

Otherwise numpy; os_namein 'posix' would be a valid specifier which it isn't

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Dependency Specifiers: Require whitespace after in and not in

Otherwise numpy; os_name in'posix' and numpy; os_name inos_name would be valid. pypa/packaging 22.0 actually allows currently numpy; os_name in'posix', but i argue there should be a whitespace after in and not in just like in python itself