shell/cli/repl.py: suppress exception context when raising exception in _execute_task

ptpython tries to execute code with 'eval', if it fails because of SyntaxError it
executes the code with 'exec'. The problem is that the SyntaxError exception
becomes part of the context of any further exception that can be raised.
So, the traceback displays a scary, unhelpful traceback with SyntaxError.
This change removes this problem, by clearing the context.
4 jobs for 818-no-stack-trace-when-using-bliss-as-a-library in 35 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #37385
check_style

00:00:19

 
  Tests
passed #37386
run_tests:source

00:30:15

32.0%
 
  Build
passed #37387
create_doc:reference

00:04:53

passed #37388
create_doc:user

00:04:41