fix: missing dependency on packaging (#1024)

This is used, but not declared. So many packages depend on this that
it's being always pulled in for testing, but in a minimal environment
it's missing.

See
pyodide/pyodide#4767 (comment).

pyproject.toml

@@ -32,7 +32,7 @@ classifiers = [
     "Operating System :: Unix",
     "Operating System :: MacOS",
 ]
-dependencies = ["numpy >=1.21"]
+dependencies = ["packaging", "numpy >=1.21"]
 
 [project.urls]
 repository = "http://github.com/scikit-hep/iminuit"