fix(deps): resolve packaging conflict with pip-audit on Python 3.7

pip-audit requires packaging>=21.0 on all Python versions, but we were
forcing Python 3.7 to use packaging>=20.0,<21.0, creating an impossible
dependency conflict during installation.

Changes:
- Remove Python 3.7-specific packaging<21.0 constraint
- Unify Python 3.7-3.9 to use packaging>=21.0,<22.0
- Add explanatory comment about pip-audit requirement
- packaging 21.x supports Python 3.7+ (only dropped 3.6)

Fixes Python 3.7 Docker test failures in CI.

Author: 1minds3t

pyproject.toml

@@ -45,9 +45,8 @@ dependencies = [
     # Filelock: vendored for <3.10, external for >=3.10 (CVE-2025-68146)
     "filelock>=3.20.1; python_version >= '3.10'",
 
-    # Packaging version splits
-    "packaging>=20.0,<21.0; python_version < '3.8'",
-    "packaging>=21.0,<22.0; python_version >= '3.8' and python_version < '3.10'",
+    # Packaging version splits - CRITICAL: pip-audit needs >=21.0 on ALL versions
+    "packaging>=21.0,<22.0; python_version < '3.10'",
     "packaging>=23.0; python_version >= '3.10'",
 
     # tomli backport for Python < 3.11
@@ -77,7 +76,7 @@ dependencies = [
     "safety>=3.7.0; python_version >= '3.10' and python_version < '3.14'",
     "marshmallow>=4.1.2; python_version >= '3.10'",  # CVE-2025-68480 override
 
-    # pip-audit fallback
+    # pip-audit fallback (requires packaging>=21.0 on ALL versions)
     "pip-audit; python_version < '3.9'",
     "pip-audit>=2.6.0; python_version >= '3.14'",