CLN: unify numpy.random-related imports
#### Location of the imports
https://github.com/pandas-dev/pandas/blob/8a25b23dd69d1e24840337797e1d20184920b5f3/pandas/tests/plotting/test_hist_method.py#L3-L4

#### Imports problem
~~All numpy-related imports are only about `numpy.random` including `randint`, `choice`, `normal`, etc~~
Part of numpy.random-related imports use aliases

#### Suggested fix for imports
eg: ~~`import numpy as np ` && `np.ranom.randn` --> `from numpy.random import randn`~~
`from numpy.random import randn` --> `import numpy as np ` && `np.ranom.randn`