get_usgs_data throws error

[sverhoeven]

I tried to run the example code at https://ewatercycle.readthedocs.io/en/latest/autoapi/ewatercycle/observation/grdc/index.html#ewatercycle.observation.grdc.get_grdc_data

But got an error

>>> from ewatercycle.observation.usgs import get_usgs_data
>>> data = get_usgs_data('03109500', '2000-01-01', '2000-12-31', cache_dir='.')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/stefanv/git/eWaterCycle/ewatercycle/src/ewatercycle/observation/usgs.py", line 126, in get_usgs_data
    ds.to_netcdf(netcdf)
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/core/dataset.py", line 2298, in to_netcdf
    return to_netcdf(  # type: ignore  # mypy cannot resolve the overloads:(
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/backends/api.py", line 1339, in to_netcdf
    dump_to_store(
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/backends/api.py", line 1386, in dump_to_store
    store.store(variables, attrs, check_encoding, writer, unlimited_dims=unlimited_dims)
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/backends/common.py", line 393, in store
    variables, attributes = self.encode(variables, attributes)
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/backends/common.py", line 482, in encode
    variables, attributes = cf_encoder(variables, attributes)
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/conventions.py", line 795, in cf_encoder
    new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()}
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/conventions.py", line 795, in <dictcomp>
    new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()}
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/conventions.py", line 199, in encode_cf_variable
    var = ensure_dtype_not_object(var, name=name)
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/conventions.py", line 154, in ensure_dtype_not_object
    data = _copy_with_dtype(data, dtype=_infer_dtype(data, name))
  File "/home/stefanv/mambaforge/envs/ewatercycle/lib/python3.10/site-packages/xarray/conventions.py", line 80, in _infer_dtype
    raise ValueError(
ValueError: unable to infer dtype on variable 'time'; xarray cannot serialize arbitrary Python objects
>>>