Address numpy deprecations

Running `pytest -vv` gives the following summary of warnings, which it would be good to address:

```
~/conda/envs/iris3/lib/python3.7/site-packages/iris/fileformats/_ff.py:819
~/conda/envs/iris3/lib/python3.7/site-packages/iris/fileformats/_ff.py:819: 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
def _parse_binary_stream(file_like, dtype=np.float, count=-1):

~/conda/envs/iris3/lib/python3.7/site-packages/pyke/knowledge_engine.py:28
~/conda/envs/iris3/lib/python3.7/site-packages/pyke/knowledge_engine.py:28: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
import imp

tests/test_cube_equaliser.py: 105 warnings
tests/test_cube_help.py: 243 warnings
tests/test_cube_loader.py: 79 warnings
~/conda/envs/iris3/lib/python3.7/site-packages/iris/fileformats/netcdf.py:439: DeprecationWarning: `np.bool` is a deprecated alias for the builtin `bool`. To silence this warning, use `bool` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.bool_` here.
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
var = variable[keys]

tests/test_cube_equaliser.py: 72 warnings
tests/test_cube_help.py: 180 warnings
tests/test_cube_loader.py: 66 warnings
~/conda/envs/iris3/lib/python3.7/site-packages/iris/fileformats/cf.py:186: DeprecationWarning: `np.bool` is a deprecated alias for the builtin `bool`. To silence this warning, use `bool` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.bool_` here.
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
return self.cf_data.__getitem__(key)

```

Start by running `pytest -vv` and compare the warning summary with the locations reported in `iris/fileformats/_ff.py`, `iris/fileformats/netcdf.py`, `iris/fileformats/cf.py`, and `pyke/knowledge_engine.py`. Determine which warnings are addressed within the project and verify that the listed NumPy and import deprecation warnings no longer appear in the test output.

Written by the indexing model from the issue text.