Fix c interface

[oleksandr-pavlyk]

One of DPCTLSyclInterface function was deprecated in #876, and __attributes__((deprecated(what, repl)) was used.

This broke building of C extensions using the library. This PR uses [[deprecated(msg)]] instead, which is understood by both the gcc and icx.

An example of native Python extension written in C and that is using DPCTLSyclInterface is added. It is being compiled in the workflow conda-package.yml workflow when testing that examples can be compiled on Linux.

Incidentally, dpctl.lsplatform implementation replaced informational print with seemingly more appropriate use of warnings.warn.

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@mingjie-intel

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/879/index.html

[coveralls]

Coverage increased (+0.1%) to 81.828% when pulling ed13b2e on fix-c-interface into b68b1e4 on master.

[mingjie-intel]

Tested with numba-dpex, with the fix, it can be built correctly.

[github-actions]

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