Fix Numpy 1.20 Deprecation Warning in cocoeval.py

[dketterer]

When I run the Python evaluation with Numpy 1.20 I get this DeprecationWarning in cocoeval.py line 378 and 379:

/pycocotools/cocoeval.py:378: 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
  tp_sum = np.cumsum(tps, axis=1).astype(dtype=np.float)

Please see the Numpy Documentation for details.

The np.float should exchanged for float.

[sadra-barikbin]

@tylin , could you please fix this issue? I made the PR. In higher versions of Numpy and python, this actually raises error.