Fix parentheses for arbitrary operator

[gmmephisto]

Closes: #336

[uranusjr]

Note that this would break things like name ===1.0;1 and name ===1.0(1), both are valid (non-PEP-440) specifiers. They are likely uncommon, and would be a good compromise to fix #336 IMO without introducing too much complexity. But it may be worthwhile to include a comment explaining the caveats.

[brettcannon]

I've asked over on the issue people like the solution this PR proposes.

[brettcannon]

Now that we have a handcrafted parser, I'm going to close this as that's probably the best way to support this situation. Thanks for at least trying, @gmmephisto .