use numpy 1.20.0 and later

[TomonobuTsujikawa]

Currently, we don't specify numpy version, so the following error occurs when system has numpy v1.19.5.

Traceback (most recent call last):
  File "classification.py", line 23, in <module>
    import nnabla as nn
  File "lib/python3.7/site-packages/nnabla/__init__.py", line 32, in <module>
    from .variable import Variable, Context
  File "lib/python3.7/site-packages/nnabla/variable.py", line 17, in <module>
    from ._variable import Context
  File "_variable.pyx", line 1, in init nnabla._variable
  File "_nd_array.pyx", line 1, in init nnabla._nd_array
ValueError: numpy.ndarray size changed, may indicate binary incompatibility. Expected 88 from C header, got 80 from PyObject
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 953, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "lib/python3.7/site-packages/nnabla/__init__.py", line 32, in <module>
    from .variable import Variable, Context
  File "lib/python3.7/site-packages/nnabla/variable.py", line 17, in <module>
    from ._variable import Context
  File "_variable.pyx", line 1, in init nnabla._variable
  File "_nd_array.pyx", line 1, in init nnabla._nd_array
ValueError: numpy.ndarray size changed, may indicate binary incompatibility. Expected 88 from C header, got 80 from PyObject

So, explicitly require numpy v1.20.0 and later when installing nnabla.