Fix chop_threshold crashes in the pandas-free complex repr - #620

On Windows with python-suitesparse-graphblas older than 7.4.3.1 the CFFI
bindings are built without complex support (MSVC lacks C99 _Complex), so
dtypes._supports_complex is False, FC64 is never registered, and
Vector.from_coo(..., dtype="FC64") raises ValueError before the test
gets to the formatting code under test. Guard with the same
skipif("not dtypes._supports_complex") pattern test_scalar.py uses;
the module already imports dtypes, which the string condition needs.

Verified the test still runs and passes on a complex-capable host, and
that the skipif string evaluates to True in the module namespace when
_supports_complex is flipped to False.

test_chop_threshold_complex failed on the macos-latest job that drew numpy
1.24.4:

    RuntimeWarning: overflow encountered in absolute

The pandas-free float formatter takes np.abs of the column to choose between
fixed and scientific notation. For a complex value with components near the
float max the magnitude is inf, which those checks already handle, but numpy
1.24 on arm64 also raises the FP overflow flag computing it (1.26 and 2.4 do
not), so the warning escaped from repr and the test suite's filterwarnings
setting turned it into an error. Reproduced locally with numpy 1.24.4,
pandas 2.2.3 and psg 8.2.0.1, then green with the magnitudes computed under
np.errstate(over="ignore").

pandas' FloatArrayFormatter makes the same np.abs call unguarded (2.2 and
3.0), so this departs from it only in not warning.