replace deprecated np.float with np.float32

[assafbot]

np.float was deprecated in numpy 1.20

[assafbot]

this PR fixes this bug #37

[rbgirshick]

The deprecation warning states:

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.

I'd prefer the fix to follow the message and use builtin float rather than np.float32

[thanhmvu]

I also got AttributeError: module 'numpy' has no attribute 'float' with np.float. Switching to float fixed it.