Improve error for local version label with unsupported operators

[pradyunsg]

This makes it clearer why the failure happens the way it happened.

Before:

packaging.requirements.InvalidRequirement: Expected end or semicolon (after version specifier)
    foo >= 1.0+local
        ~~~~~~^

After:

packaging.requirements.InvalidRequirement: Local version label can only be used with `==` or `!=` operators
    foo >= 1.0+local
        ~~~~~~^