Refactor setup requirements

requirements/dev.txt

@@ -7,5 +7,4 @@ isort>=4.2
 flake8>=3.5.0
 flake8-isort>=2.3
 
-# do not unpin this (see setup.py)
-setuptools_scm==1.15.6
+-r setup.txt

requirements/setup.txt

@@ -0,0 +1,4 @@
+# requirements for building the distributable package
+
+# do not unpin this (see setup.py)
+setuptools_scm==1.15.6