Manually install setuptools-scm for Travis deploy

This only works because we still mostly use default build configuration of setuptools and wheel, so we don't really depend on PEP517 builds, only PEP518 build requirements.

.travis.yml

@@ -25,6 +25,10 @@ jobs:
 
     - stage: publish
       python: '3.6'
+      before_script:
+        # workaround for Travis' inability to build PEP517 projects; anything added to build-system.requires
+        # will also have to be added here until Travis implements this
+        - pip install setuptools-scm
       script: skip
       env: PYPI_DEPLOY=true
       deploy: &pypi