pkg: remove unneeded and obsolete mock requirement

the mock library is obsolete and not even needed, so remove it from the
test requirements

setup.py

@@ -98,7 +98,7 @@ def list_modules(dirname):
     requires=["falcon", "requests"],
     install_requires=["falcon==2.0.0", "requests"],
     setup_requires=["pytest-runner"],
-    tests_require=["pytest", "mock", "marshmallow"],
+    tests_require=["pytest", "marshmallow"],
     ext_modules=ext_modules,
     cmdclass=cmdclass,
     python_requires=">=3.5",