MNT: Replace fromstring with frombuffer, bump numpy minversion

[pllim]

I see this warning while working on astropy/astrowidgets#125

.../ginga/aggw/CanvasRenderAgg.py:215: DeprecationWarning: The binary mode of fromstring is deprecated,
as it behaves surprisingly on unicode inputs. Use frombuffer instead
    arr8 = np.fromstring(self.surface.tobytes(), dtype=np.uint8)

[ejeschke]

@pllim , what version of numpy deprecates fromstring? I think there are a few instances where ginga checks for frombuffer and falls back to fromstring (and similar for tobuffer/tostring ?). If we are only supporting numpy versions that have frombuffer/tobuffer then we can modify those sections too.

[pllim]

If I understand https://numpy.org/doc/stable/reference/generated/numpy.fromstring.html correctly, it has been deprecated since 1.14. I am not familiar with tostring but these are all the fromstring I found in Ginga.

[ejeschke]

Then.., currently setup.cfg says that we require:

install_requires = numpy>=1.13

maybe we should bump that to 1.14 in this PR? What do you think?

I can submit a separate PR to tackle the other cases, if I find any.

[pllim]

1.14 is pretty old. Even astropy only support numpy>=1.17 now.

[ejeschke]

@plllim, shall I SAM (Squash And Merge)? 😃 Or are you planning to push more commits?

[pllim]

@ejeschke , feel free to SAM. I am done here. Thanks! 😸