AttributeError when converts IF to json

I built the repo and ran the command trying to convert a physical netlist IF to a .json file:
```python3 -m fpga_interchange.convert \
--schema_dir "/home/local/eda15/zhilix/projects/RapidWright/interchange/fpga-interchange-schema/interchange" \
--schema physical \
--input_format capnp \
--output_format json \
FPGA01_postroute_interchange.phys \
FPGA01_postroute_interchange.json
```
But I got error message like this:
```Traceback (most recent call last):
File "/home/polaris/zhilix/.pyenv/versions/3.8.4/lib/python3.8/runpy.py", line 194, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/home/polaris/zhilix/.pyenv/versions/3.8.4/lib/python3.8/runpy.py", line 87, in _run_code
exec(code, run_globals)
File "/home/local/eda15/zhilix/projects/python-fpga-interchange/fpga_interchange/convert.py", line 208, in
main()
File "/home/local/eda15/zhilix/projects/python-fpga-interchange/fpga_interchange/convert.py", line 201, in main
message = read_format(schema, args.input_format, in_f)
File "/home/local/eda15/zhilix/projects/python-fpga-interchange/fpga_interchange/convert.py", line 105, in read_format
message = message.as_builder()
AttributeError: '_GeneratorContextManager' object has no attribute 'as_builder'
```
This is due to the from_bytes() in pycapnp library:
[https://github.com/capnproto/pycapnp/issues/287](url)