[BUG] ci: windows 2016, error in `test_str_leak`

Observation

In this job on master: https://github.com/pybind/pybind11/runs/1128209634#step:17:33

      @pytest.mark.xfail("env.PYPY")
      def test_str_leak():
...
          pytest.gc_collect()
  >       assert getrefcount(fmt) == start
  E       AssertionError: assert 10 == 11
  E        +  where 10 = <built-in function getrefcount>('f4')

Speculation

It looks like there may be an ABI mismatch? (e.g. pybind11s ABI shim is out of date for this version or something?)

     C:\hostedtoolcache\windows\Python\3.7.9\x86\lib\importlib\_bootstrap.py:219: RuntimeWarning: numpy.ufunc size changed, may indicate binary incompatibility. Expected 112 from C header, got 124 from PyObject

Start with the Windows 2016 CI run and the failing test_str_leak test, then compare its refcount assertion with the reported NumPy ABI warning. Done means determining whether the binary incompatibility explains the failure and restoring a passing Windows job.