add skip existing to python workflow

Author: andrewfulton9

.github/workflows/python.yml

@@ -29,4 +29,5 @@ jobs:
       with:
         user: __token__
         password: ${{ secrets.UPATH_GIT_REPO }}
-        verbose: true
+        verbose: true
+        skip_existing: true

noxfile.py

@@ -18,7 +18,7 @@ def smoke(session):
     session.run(*"pytest --skiphdfs upath".split())
 
 
-@nox.session(python=False)
+@nox.session(python=['3.7', '3.8'])
 def build(session):
     session.install("flit")
     session.run(*"flit build".split())