in the end it generates the bitstream but I'm still getting this:
```
Writing bitstream ...
/opt/conda/envs/xc7/lib/python3.7/site-packages/fasm/parser/__init__.py:33: RuntimeWarning: Unable to import fast Antlr4 parser implementation.
  ImportError: cannot import name 'antlr_to_tuple' from 'fasm.parser' (/opt/conda/envs/xc7/lib/python3.7/site-packages/fasm/parser/__init__.py)

  Falling back to the much slower pure Python textX based parser
  implementation.

  Getting the faster antlr parser can normally be done by installing the
  required dependencies and then reinstalling the fasm package with:
    pip uninstall
    pip install -v fasm

  """.format(e), RuntimeWarning)
```