binwalk: migrate to python@3.9

[fxcoudert]

As part of the Python 3.9 migration (#62201).

This formula is independent from the all other Python formulas (if I didn't screw up my script or my logic)

Do not merge before the next Brew tag ships, expected on Monday 2020-10-12

[fxcoudert]

    RuntimeError: Polyfit sanity test emitted a warning, most likely due to using a buggy Accelerate backend. If you compiled yourself, see site.cfg.example for information. Otherwise report this to the vendor that provided NumPy.
    RankWarning: Polyfit may be poorly conditioned

[yous]

resource "numpy" specifies numpy-1.18.2, but the error message came from numpy-1.19.2: /private/tmp/pip-req-build-r5g_s2kr/.eggs/numpy-1.19.2-py3.9-macosx-10.15-x86_64.egg/numpy/__init__.py

[yous]

Changing the order of resource "numpy" and resource "matplotlib" works, as matplotlib requires numpy. To build numpy 1.19.2, numpy should be configured with OpenBLAS or other backends instead of Accelerate: numpy/numpy#15947