pybind11 type caster for sycl::half

[ndgrigorian]

This pull request proposes removal of PythonObjectUnboxer in favor of a pybind11 type caster for sycl::half. This permits extensions to convert Python scalars to sycl::half with only the inclusion of dpctl4pybind11.hpp.

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 88.218%. remained the same
when pulling 3f1a40d on type-caster-sycl-half
into aaf444e on master.

[github-actions]

Array API standard conformance tests for dpctl=0.17.0dev0=py310h15de555_312 ran successfully.
Passed: 870
Failed: 8
Skipped: 92

[github-actions]

Array API standard conformance tests for dpctl=0.17.0dev0=py310h15de555_313 ran successfully.
Passed: 870
Failed: 8
Skipped: 92

[github-actions]

Array API standard conformance tests for dpctl=0.17.0dev0=py310h15de555_314 ran successfully.
Passed: 870
Failed: 8
Skipped: 92