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

Makefile

@@ -18,7 +18,9 @@ bumpdeps:
 	python tools/bumpdeps.py
 
 # Development environment
-setupenv:
+newenv:
 	python3.7 -m venv --clear .venv
 	.venv/bin/pip install -U pip setuptools
-	.venv/bin/pip install -Ur dev-requirements.txt
+	$(MAKE) syncenv
+syncenv:
+	.venv/bin/pip install -Ur ./tools/dev-requirements.txt