…12136)

This combines `get_normals` and `_generate_normals`, and eliminates all other calls to np.cross.

`get_normals` and `_generate_normals` were profiled, and it was found that vectorizing `np.cross`  like in `get_normals` was faster:

```python
import numpy as np

def get_normals(polygons):
    v1 = np.empty((len(polygons), 3))
    v2 = np.empty((len(polygons), 3))
    for poly_i, ps in enumerate(polygons):
        # pick three points around the polygon at which to find the
        # normal doesn't vectorize because polygons is jagged
        i1, i2, i3 = 0, len(ps)//3, 2*len(ps)//3
        v1[poly_i, :] = ps[i1, :] - ps[i2, :]
        v2[poly_i, :] = ps[i2, :] - ps[i3, :]
    return np.cross(v1, v2)

def _generate_normals(self, polygons):
    normals = []
    for verts in polygons:
        v1 = np.array(verts[0]) - np.array(verts[1])
        v2 = np.array(verts[2]) - np.array(verts[0])
        normals.append(np.cross(v1, v2))
    return np.array(normals)

polygons = [
    np.random.rand(np.random.randint(10, 1000), 3)
    for i in range(100)
]

%timeit _generate_normals(polygons)
# 3.14 ms ± 255 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

%timeit get_normals(polygons)
# 452 µs ± 4.33 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
```

* upstream/master: (1723 commits)
  Correctly get weight & style hints from certain newer Microsoft fonts (matplotlib#12945)
  Remove some checks for Py<3.6 in the test suite. (matplotlib#12974)
  Fail-fast when trying to run tests with too-old pytest.
  Include scatter plots in Qt figure options editor. (matplotlib#12779)
  ENH: replace deprecated numpy header
  Minor simplifications.
  tickminorvisible-fix (matplotlib#12938)
  Remove animated=True from animation docs
  Update the documentation of Cursor
  Misc. cleanups.
  Add test for 3d conversion of empty PolyCollection
  Support ~ as nonbreaking space in mathtext.
  Deprecate public use of Formatter.pprint_val.
  MAINT: Unify calculation of normal vectors from polygons (matplotlib#12136)
  Fix the title of testing_api
  More table documentation
  Simplify bachelors degree example using new features.
  Avoid pyplot in showcase examples.
  Simplify argument checking in Table.__getitem__. (matplotlib#12932)
  Minor updates following bump to Py3.6+.
  ...

# Conflicts:
#	lib/matplotlib/widgets.py