Use pip install . instead of calling setup.py (#5442)

It is almost the same but is more consistent and future proof

Author: Carreau

docs/source/install.rst

@@ -28,7 +28,7 @@ To install distributed from source, clone the repository from `github
 
     git clone https://github.com/dask/distributed.git
     cd distributed
-    python setup.py install
+    python -m pip install .
 
 
 Notes