Remove deprecated use of numpy aliases of built-in types

[rly]

Using datajoint 0.13.2 and numpy 1.20.3 or the latest 1.21.1, I get the following deprecation warning repeatedly:

/home/rly/miniconda3/envs/nwb_datajoint/lib/python3.8/site-packages/datajoint/table.py:618: DeprecationWarning: `np.float` is a deprecated alias for the builtin `float`. To silence this warning, use `float` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.float64` here.
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
  if value is None or (attr.numeric and (value == '' or np.isnan(np.float(value)))):

See numpy deprecation info here: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations

There are several uses of np.float, np.bool, np.object in the codebase that raise the same deprecation warning above.