Add uvloop as Posix+CPython dependency and tweak new Make recipes (#2819)

- uvloop is now a dependency on non-Windows CPython systems
- `make setupenv` renamed to `make newenv`
- `make syncenv` added to sync local venv to current dependencies
- `dev-requirements.txt` moved into `tools` directory

tools/dev-requirements.txt

@@ -0,0 +1,3 @@
+packaging
+tox
+-e .[docs,mongo,style,test]

tools/primary_deps.ini

@@ -19,6 +19,7 @@ install_requires =
     Red-Lavalink
     schema
     tqdm
+    uvloop; sys_platform != "win32" and platform_python_implementation == "CPython"
     # Websockets is a secondary dependency, but until pip has a complete dependency resolver, we
     # need to list it here to avoid an incompatible version being installed.
     # See under point 2 here: https://pip.pypa.io/en/stable/user_guide/#requirements-files