open_dataset regression

[dcherian]

What happened:

open_dataset doesn't replace ~ any more

What you expected to happen:

should replace ~ for backward compatibility

Minimal Complete Verifiable Example:

import xarray as xr

da = xr.DataArray([1, 2, 3])
da.to_netcdf("~/bug_report.nc")

xr.open_dataarray("~/bug_report.nc")

Anything else we need to know?:
works on 0.17.0, fails on master

[aurghs]

I think that this is consequence of the refactor done by @alexamici when he has removed _normalize_path: #4701
We decided to demand the path interpretation to the backends.

I'll have a look to understand how to fix it.

[alexamici]

The reason to delegate all path normalisation to backends is that not all strings are paths, for example 'NETCDF:"~/sst.nc":tos' is a valid source string for GDAL / rasterio.

I would suggest to add path normalisation to the backends that needs them, in order to avoid any special case in the common code path.

@aurghs I suggest you add os.path.expanduser in the netCDF backend.