Install Python3 only dev tools with tools/venv3.py (#7215)

These packages can be useful and I found that they aren't being installed in our Python 3 development environment. Let's fix that.

tools/venv.py

@@ -25,6 +25,10 @@ def main(pip_args=None):
 
     venv_path = _venv_common.prepare_venv_path('venv')
     create_venv(venv_path)
+
+    if not pip_args:
+        pip_args = _venv_common.REQUIREMENTS
+
     _venv_common.install_packages(venv_path, pip_args)