Engine is no longer inferred for filenames not ending in ".nc"

[shoyer]

This works with xarray=0.17.0:

import xarray
xarray.Dataset({'x': [1, 2, 3]}).to_netcdf('tmp')
xarray.open_dataset('tmp')

On xarray 0.18.0, it fails:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-20e128a730aa> in <module>()
      2 
      3 xarray.Dataset({'x': [1, 2, 3]}).to_netcdf('tmp')
----> 4 xarray.open_dataset('tmp')

/usr/local/lib/python3.7/dist-packages/xarray/backends/api.py in open_dataset(filename_or_obj, engine, chunks, cache, decode_cf, mask_and_scale, decode_times, decode_timedelta, use_cftime, concat_characters, decode_coords, drop_variables, backend_kwargs, *args, **kwargs)
    483 
    484     if engine is None:
--> 485         engine = plugins.guess_engine(filename_or_obj)
    486 
    487     backend = plugins.get_backend(engine)

/usr/local/lib/python3.7/dist-packages/xarray/backends/plugins.py in guess_engine(store_spec)
    110             warnings.warn(f"{engine!r} fails while guessing", RuntimeWarning)
    111 
--> 112     raise ValueError("cannot guess the engine, try passing one explicitly")
    113 
    114 

ValueError: cannot guess the engine, try passing one explicitly

I'm not entirely sure what changed. My guess is that we used to fall-back to trying to use SciPy, but don't do that anymore. A potential fix would be reading strings as filenames in xarray.backends.utils.read_magic_number.

Related: #5291