Fix/code class pygments short hex #2355

[leonor-p]

Motivation

The Code class raises an exception when instantiated with any string.
PangoMarkupFormatter in pygments >= 2.14 generates 3-digit hex color
codes (e.g. #abc) in markup attributes. Pango does not support short
hex notation, causing MarkupText to fail on instantiation.

Fixes #2355

Proposed changes

• Added regex substitution in Code.__init__ in manimlib/mobject/svg/text_mobject.py that expands all 3-digit hex colors to 6-digit after pygments.highlight(), before passing markup to MarkupText
• Added unit tests in tests/test_code_class.py verifying the regex expansion works correctly on Python 3.9

Test

Code:

c = Code('sorted(iterable, key=None, reverse=False)')

Result: Code object created successfully without exception.
Unit tests pass on Python 3.9.25 — 2 passed in 0.02s.