Add setuptools as a dev dependency

It is necessary for building the package when running the integration tests,
but more recent Python versions no longer include it by default:
python/cpython#101039.

requirements/dev.txt

@@ -5,6 +5,7 @@ pytest-cov>=2.4.0
 pylint>=1.7.2
 black>=20.8b0
 bandit>=1.6.2
+setuptools
 
 # Test requirements
 pytest>=3.0.7