sphinx-autoapi v3.2.1 generates duplicate object warnings

[bkeryan]

          The new version of sphinx-autoapi is producing more warnings about duplicate objects.

/home/runner/work/measurement-plugin-python/measurement-plugin-python/packages/service/_docs_source/autoapi/ni_measurement_plugin_sdk_service/measurement/service/index.rst:138: WARNING: duplicate object description of ni_measurement_plugin_sdk_service.measurement.service.MeasurementService.measurement_info, other instance in autoapi/ni_measurement_plugin_sdk_service/measurement/service/index, use :no-index: for one of them
/home/runner/work/measurement-plugin-python/measurement-plugin-python/packages/service/_docs_source/autoapi/ni_measurement_plugin_sdk_service/measurement/service/index.rst:144: WARNING: duplicate object description of ni_measurement_plugin_sdk_service.measurement.service.MeasurementService.service_info, other instance in autoapi/ni_measurement_plugin_sdk_service/measurement/service/index, use :no-index: for one of them
/home/runner/work/measurement-plugin-python/measurement-plugin-python/packages/service/_docs_source/autoapi/ni_measurement_plugin_sdk_service/measurement/service/index.rst:147: WARNING: duplicate object description of ni_measurement_plugin_sdk_service.measurement.service.MeasurementService.context, other instance in autoapi/ni_measurement_plugin_sdk_service/measurement/service/index, use :no-index: for one of them

Originally posted by @bkeryan in #810 (comment)

AB#2817983

[bkeryan]

Here's what I think is happening:

• The class documents these attributes in the class docstring using the Napoleon "Attributes:" keyword.
• sphinx-autoapi also sees these attributes when it parses the code of the __init__ method.
• Previously, sphinx-autoapi silently ignored undocumented members in some cases: Undocumented instance attributes are ignored even with "undoc-members" selected readthedocs/sphinx-autoapi#451

Removing the Napoleon "Attributes:" keyword and adding docstrings after the attribute assignments seems to be a reasonable way to fix this.