"stub" docstrings include "special cases":

> `if x_i is +infinity, sqrt(x_i) is +infinity` etc

`test_special_cases` parses these statements from docstrings and makes them
into tests cases.

So far, parsing only worked for real value cases, and failed for complex-valued
cases:

> For complex floating-point operands, let a = real(x_i), b = imag(x_i), and
> `If a is either +0 or -0 and b is +0, the result is +0 + 0j.`

These stanzas simply generate "case for {func} is not machine-readable" `UserWarning`s.
Quite a wall of them.

Therefore, we update parsing and testing code to take these complex-valued
cases into accout.
For now, we only consider unary functions.

The effect is:

$ ARRAY_API_TESTS_MODULE=array_api_compat.torch pytest array_api_tests/test_special_cases.py::test_unary

generates
- "128 passed, 177 warnings in 0.78s" on master
- "11 failed, 241 passed, 49 warnings in 1.82s" on this branch

So that there are new failures (from new complex-valued cases) but we 128 less warnings.

- Update r_complex_value regex to match ± in sign positions
- Normalize ± to + in parse_complex_value() function
- Detect ± symbols in parse_complex_result() to set unspecified sign flags
- Results: 246 passed (+4), 45 warnings (-4)

Handle spaces after ± symbol in complex value parsing

Update regex to allow optional whitespace between sign and value: ([±+-]?)\s*
This now handles cases like "± 0 + 0j" in addition to "±0 + 0j"