Fix python call syntax highlighting

[pencels]

Summary

Fixes an issue with the Python lexer where any identifier used before an opening paren ( would be highlighted as a function name. This led to inconsistent highlighting in the case of for loops, where one for would be rendered as a keyword while the other was rendered as a function name:

This pull request also addresses other edge cases for call expressions:

• distinguishes Exception classes from normal classes in a call expr context
• distinguishes Builtin calls from identically named method calls

Visual Before and After

Before	After