Add support for PEP-515, underscores in numeric literals

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Adds support for separating numbers with underscores, per PEP-515.

Also adds specs for numbers.

Alternate Designs

None, this is a PEP.

Benefits

Proper syntax highlighting! And regression tests!

Possible Drawbacks

People using Python versions < 3.6 will also get this highlighting, even though the PEP is only applicable to >= 3.6.

Applicable Issues

Fixes #221.