ENH: replace uses of np.isscalar with pd.lib.isscalar
Closes #8708

Added note that pd.lib.isscalar() returns True for None, which is different from np.isscalar().

I don't see any tests for `lib.isscalar` itself. Can you add some in `test_common.py` (and of course include `None` for this)
 Tests for `lib.isscalar` are already in `test_lib.py`.

https://github.com/pydata/pandas/blob/master/pandas/tests/test_lib.py#L189

Do you want these moved to `test_common.py`?
 @chrisaycock thanks!

FYI, build was failing because of some flake warnings.
