Fix TypeError when retracting analysis with ExtendedField

[xispa]

Description of the issue/feature this PR addresses

SchemaExtender fields don't auto-generate the accessor/mutator methods, so an error is raised when trying to retract a given analysis with an ExtendedField because the system tries to copy the field values to the retest.

Current behavior before PR

Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.lims.browser.workflow, line 143, in __call__
  Module bika.lims.browser.workflow, line 154, in __call__
  Module bika.lims.browser.workflow, line 173, in do_action
  Module bika.lims.workflow, line 127, in doActionFor
  Module Products.CMFCore.WorkflowTool, line 241, in doActionFor
  Module Products.CMFCore.WorkflowTool, line 552, in _invokeWithNotification
  Module Products.DCWorkflow.DCWorkflow, line 282, in doActionFor
  Module Products.DCWorkflow.DCWorkflow, line 421, in _changeStateOf
  Module Products.DCWorkflow.DCWorkflow, line 531, in _executeTransition
  Module zope.event, line 31, in notify
  Module zope.component.event, line 24, in dispatch
  Module zope.component._api, line 136, in subscribers
  Module zope.component.registry, line 321, in subscribers
  Module zope.interface.adapter, line 585, in subscribers
  Module zope.component.event, line 32, in objectEventNotify
  Module zope.component._api, line 136, in subscribers
  Module zope.component.registry, line 321, in subscribers
  Module zope.interface.adapter, line 585, in subscribers
  Module bika.lims.workflow, line 197, in AfterTransitionEventHandler
  Module bika.lims.workflow, line 172, in call_workflow_event
  Module bika.lims.workflow.analysisrequest.events, line 77, in after_retract
  Module bika.lims.workflow.analysisrequest, line 52, in do_action_to_analyses
  Module bika.lims.workflow, line 127, in doActionFor
  Module Products.CMFCore.WorkflowTool, line 241, in doActionFor
  Module Products.CMFCore.WorkflowTool, line 552, in _invokeWithNotification
  Module Products.DCWorkflow.DCWorkflow, line 282, in doActionFor
  Module Products.DCWorkflow.DCWorkflow, line 421, in _changeStateOf
  Module Products.DCWorkflow.DCWorkflow, line 531, in _executeTransition
  Module zope.event, line 31, in notify
  Module zope.component.event, line 24, in dispatch
  Module zope.component._api, line 136, in subscribers
  Module zope.component.registry, line 321, in subscribers
  Module zope.interface.adapter, line 585, in subscribers
  Module zope.component.event, line 32, in objectEventNotify
  Module zope.component._api, line 136, in subscribers
  Module zope.component.registry, line 321, in subscribers
  Module zope.interface.adapter, line 585, in subscribers
  Module bika.lims.workflow, line 197, in AfterTransitionEventHandler
  Module bika.lims.workflow, line 172, in call_workflow_event
  Module bika.lims.workflow.analysis.events, line 144, in after_retract
  Module bika.lims.utils.analysis, line 95, in create_analysis
  Module bika.lims.utils.analysis, line 78, in copy_analysis_field_values
TypeError: getattr(): attribute name must be string

Desired behavior after PR is merged

No error. The retest is created and the value for the extended field kept in the retest.

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.