python domain: support unary subtraction operator (ast.USub) during annotation parsing

[jayaddison]

Feature or Bugfix

• Bugfix

Purpose

• Allow negative literal integers expressed using the unary subtraction operator when parsing Python type annotations.

Detail

• Unary addition is not supported since literals such as +2 resolves to 2.
• Unusual edge cases could remain - this does not attempt to handle foo[1+2j] for example -- but that seems unlikely to be a likely annotation (?).
• Strictly speaking, does not add test coverage for the intersphinx case mentioned in the test bugreport; but I believe that these changes will solve the problem.

Relates

• Resolves Reference target not found for typing.Literal with negative integer argument #11900.

[nulano]

Thank you, I can confirm the reference is resolved with sphinx installed from master.

[jayaddison]

You're welcome - thanks for taking the time to check and confirm the results!