Dagstermill Windows paths & JSON Decode error

[gherka]

Just to flag up that injecting parameters into Jupyter notebooks on Windows throws up a JSONDecodeError. It looks like this is happening because os.path.join will use the \\ separator so that the path looks like this "/tmp/dagstermill/c18e8d32-b138-46c6-8f8e-1efbc5a02668/marshal\\input-name". The first part is hard-coded (Line 99 of solids.py) with forward slashes and then os.path.join adds Windows-specifc \\ that doesn't get escaped. Changing Line 127 in solids.py to only use "/" fixes the issue for me in the short term, but it's probably not the best solution:

parameter_value = write_value(
  # dagster_type, input_value, os.path.join(marshal_dir, "input-{}".format(input_name))
  dagster_type, input_value, "/".join([marshal_dir, "input-{}".format(input_name)])
)