chore(deps): remove redundant Pygments pin from requirements.txt (closes #17)

Application source has zero `pygments` references — confirmed via
grep across all .py / .html / .js / .css. The explicit
`Pygments==2.19.2` pin is residue from a stale pip-freeze snapshot.

Notable: pytest 9.x lists pygments as a transitive `Requires:`
(unlike pytest 8.x where it was optional/extra), so a fresh
`pip install -r requirements.txt` will still install pygments via
the pytest dep tree. Removing the explicit pin is therefore a
"drop a redundant version pin" change, not a "remove an installed
package" change. The benefit: future pytest bumps that bring a
different pygments range no longer fight an unrelated explicit pin.

Verification (clean venv built from this requirements.txt):
- pip install -r requirements.txt: succeeds; pygments 2.20.0 pulled
  transitively by pytest, as expected.
- pytest tests/ : 75/75 OK (was 75/75 before).
- python app.py + curl /: HTTP 200.

Author: timon0305

requirements.txt

@@ -9,7 +9,6 @@ Jinja2==3.1.6
 MarkupSafe==3.0.3
 packaging==26.0
 pluggy==1.6.0
-Pygments==2.19.2
 pytest==9.0.2
 tomli==2.4.0
 typing_extensions==4.15.0