Useful to avoid full rebuild just to test adding a new package or
upgrading an existing package.

Sort of a restore of commit cbfa5d5.

…nt build

Previous build probably got pydantic via indirect dependency but somehow
this has changed so now have to add pydantic explicitly, for Raven notebooks.

The fix did not make it to RavenPy 0.5.1.

To fix error running Raven notebooks:
```
  ---------------------------------------------------------------------------
  ImportError                               Traceback (most recent call last)
  <ipython-input-1-021da278d129> in <module>
        2
        3 from birdy import WPSClient
  ----> 4 from ravenpy.utilities.testdata import get_file
        5 import os
        6 import xarray as xr

  /opt/conda/envs/birdy/lib/python3.7/site-packages/ravenpy/utilities/testdata.py in <module>
       14 from xarray import Dataset
       15 from xarray import open_dataset as _open_dataset
  ---> 16 from xarray.tutorial import file_md5_checksum
       17
       18 _default_cache_dir = Path.home() / ".raven_testing_data"

  ImportError: cannot import name 'file_md5_checksum' from 'xarray.tutorial' (/opt/conda/envs/birdy/lib/python3.7/site-packages/xarray/tutorial.py)
```

…onda-forge

…ershed_properties.ipynb

…:210415 to reproduce Raven nb prob

…10615