Replaces use of numpy's tostring() with tobytes()

[jameshiebert]

numpy has deprecated the use of array.array.tostring() in favor of
tobytes
numpy/numpy#15867
This patch replaces all (6) such uses in this package.

Fixes #1023

I haven't done extensive testing, but the test suite runs fine on my dev machine:

(venv) james@basalt:~/code/git/netcdf4-python/test$ python run_all.py 
not running tst_unicode.py ...
not running tst_create_mem.py ...
not running tst_cdf5.py ...

netcdf4-python version: 1.5.4
HDF5 lib version:       1.10.0
netcdf lib version:     4.6.0
numpy version           1.19.0
cython version          0.29.20
......../home/james/code/git/netcdf4-python/test/tst_types.py:92: UserWarning: WARNING: missing_value not used since it
cannot be safely cast to variable data type
  assert_array_equal(v3[:],-1*np.ones(n2dim,v3.dtype))
..................................................................................foo_bar
.
----------------------------------------------------------------------
Ran 91 tests in 4.479s

OK

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[jswhit]

Looks good thanks - could you just add a Changelog entry?