[REST source] fixes deprecation warning

[willi-mueller]

Tell us what you do here

• [x] improving, documenting, or customizing an existing source (please link an issue or describe below)

Relevant issue

issue #313

More PR info

Before:

See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

tests/rest_api/conftest.py:75
  dlt-verified-sources/tests/rest_api/conftest.py:75: DeprecationWarning: invalid escape sequence '\?'
    @router.get("/posts(\?page=\d+)?$")

tests/rest_api/conftest.py:79
  dlt-verified-sources/tests/rest_api/conftest.py:79: DeprecationWarning: invalid escape sequence '\d'
    @router.get("/posts/(\d+)/comments")

tests/rest_api/conftest.py:84
  dlt-verified-sources/tests/rest_api/conftest.py:84: DeprecationWarning: invalid escape sequence '\d'
    @router.get("/posts/\d+$")

tests/rest_api/conftest.py:89
  dlt-verified-sources/tests/rest_api/conftest.py:89: DeprecationWarning: invalid escape sequence '\d'
    @router.get("/posts/\d+/some_details_404")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

After

[burnash]

Thanks, @willi-mueller!