Improve error reporting when searching for dpctl.

[diptorupd]

The PR improves the user warnings that are printed when a required version of dpctl is not found. There are three things that are checked for numba_dppy to set the dppy_found flag:

1. import dpctl does not raise an error.
2. The dpctl version is at least 0.7.0.
3. The dpctl.select_default_device().is_host returns False, i.e., there is at least one non-host SYCL device.

Closes #219.

[diptorupd]

@PokhodenkoSA At least some of the UserWarning can be tested if we set these two environment variables: SYCL_DEVICE_FILTER=host and SYCL_ENABLE_HOST_DEVICE=1. Trying to import numba_dppy after setting these two environment variables will trigger the dpctl could not find any non-host SYCL device... warning. I can trigger it in normal Python. Do you know how to set environment variables in Pytest fixtures? I tried the following test case but that did not work:

import pytest
import os


def test_import_numba_dppy_with_only_host_device():
    with pytest.warns(
        UserWarning, match=r"dpctl could not find any non-host SYCL device"
    ):
        try:
            old_environ = dict(os.environ)
            os.environ["SYCL_DEVICE_FILTER"] = "host"
            os.environ["SYCL_ENABLE_HOST_DEVICE"] = "1"
            import numba_dppy
        except ImportError:
            pass
        finally:
            os.environ.clear()
            os.environ.update(old_environ)