Pin netCDF4-python version to pre 1.6

[SorooshMani-NOAA]

As of now versions 1.6 and above result in errors on GitHub actions when trying to open hamtide .nc files:

During handling of the above exception, another exception occurred:

    def test_open_w_xr():
>       ds = xr.open_dataset(url)

tests/test_github_action.py:16: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../../.local/lib/python3.8/site-packages/xarray/backends/api.py:539: in open_dataset
    backend_ds = backend.open_dataset(
../../../.local/lib/python3.8/site-packages/xarray/backends/netCDF4_.py:555: in open_dataset
    store = NetCDF4DataStore.open(
../../../.local/lib/python3.8/site-packages/xarray/backends/netCDF4_.py:384: in open
    return cls(manager, group=group, mode=mode, lock=lock, autoclose=autoclose)
../../../.local/lib/python3.8/site-packages/xarray/backends/netCDF4_.py:332: in __init__
    self.format = self.ds.data_model
../../../.local/lib/python3.8/site-packages/xarray/backends/netCDF4_.py:393: in ds
    return self._acquire()
../../../.local/lib/python3.8/site-packages/xarray/backends/netCDF4_.py:387: in _acquire
    with self._manager.acquire_context(needs_lock) as root:
/usr/lib/python3.8/contextlib.py:113: in __enter__
    return next(self.gen)
../../../.local/lib/python3.8/site-packages/xarray/backends/file_manager.py:189: in acquire_context
    file, cached = self._acquire_with_cache_info(needs_lock)
../../../.local/lib/python3.8/site-packages/xarray/backends/file_manager.py:207: in _acquire_with_cache_info
    file = self._opener(*self._args, **kwargs)
src/netCDF4/_netCDF4.pyx:24[63](https://github.com/noaa-ocs-modeling/adcircpy/actions/runs/3251212919/jobs/5335909325#step:6:64): in netCDF4._netCDF4.Dataset.__init__
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   OSError: [Errno -[68](https://github.com/noaa-ocs-modeling/adcircpy/actions/runs/3251212919/jobs/5335909325#step:6:69)] NetCDF: I/O failure: b'https://icdc.cen.uni-hamburg.de/thredds/dodsC/ftpthredds/hamtide//m2.hamtide11a.nc'

src/netCDF4/_netCDF4.pyx:2026: OSError
----------------------------- Captured stderr call -----------------------------
Note:Caching=1
Error:curl error: Problem with the SSL CA cert (path? access rights?)
curl error details: 
Warning:oc_open: Could not read url

[SorooshMani-NOAA]

See Unidata/netcdf-c#2459