fix(deps): add Pygments to requirements.txt (rich.syntax dep) (#9819)

`lib/tools/patching.py` imports `from rich.syntax import Syntax`. The
`rich` package already in requirements.txt does not pull `pygments` as a
hard dep — it is an extras_require for syntax highlighting. On a fresh
host where `requirements.txt` is the only source of pip installs, the
build fails:

  Traceback (most recent call last):
    File "/armbian/lib/tools/patching.py", line 444, in <module>
      from rich.syntax import Syntax
    File ".../rich/syntax.py", line 24, in <module>
      from pygments.lexer import Lexer
  ModuleNotFoundError: No module named 'pygments'

Pin Pygments == 2.19.1 so dependabot can keep it current alongside rich.

Assisted-by: Claude:claude-opus-4.7

Author: iav

requirements.txt

@@ -16,5 +16,6 @@ PyYAML == 6.0.3        # for parsing/writing YAML
 oras == 0.2.42         # for OCI stuff in mapper-oci-update
 Jinja2 == 3.1.6        # for templating
 rich == 15.0.0         # for rich text formatting
+Pygments == 2.19.1     # syntax highlighting, required by rich.syntax (imported by lib/tools/patching.py)
 dtschema == 2026.4     # for checking dts files and dt bindings
 yamllint == 1.38.0     # for checking dts files and dt bindings