Support for underscores in numeric literals (Python 3.6)

Summary:
This patch adds support for underscores in numeric literals for Python 3.6 (from [[ https://www.python.org/dev/peps/pep-0515/ | PEP 515 ]])

BUG: 385422

Test Plan:
Test different numbers with underscore in the code editor sample.

an_int = 100_000_000
a_float = 100_0000_000.0
some_hex = 0xEAD2_DF24_3D

Reviewers: #kate, dhaumann

Reviewed By: #kate, dhaumann

Subscribers: dhaumann, cullmann, #frameworks

Tags: #framework_syntax_highlighting, #frameworks

Differential Revision: https://phabricator.kde.org/D8371