Fix stacktrace variable side-effect

Stacktrace listener could cause side-effect failure in specific cases.
This has now been fixed by updating dependency to robotframework-stacktrace 0.4.1

Stack traces and bye bye Python 2.7

This release drops support to Python 2.7.
And adds robotframework-stacktrace when running with normal robot without listeners.

So console output will be like this:

pabot tmp
2021-06-08 21:21:23.083356 [PID:3664] [0] [ID:0] EXECUTING Tmp.Testing
2021-06-08 21:21:23.394708 [PID:3664] [0] [ID:0] Execution failed in Tmp.Testing with 1 failing test(s)
==============================================================================
Tmp
==============================================================================
Tmp.Testing
==============================================================================
Testing
  Traceback (most recent call last):
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    File  ../testing.robot:2
    T:  Testing
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    File  ../testing.robot:4
      Keyer
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    File  ../testing.robot:8
      Fail    foo
______________________________________________________________________________
| FAIL |
foo
------------------------------------------------------------------------------
Tmp.Testing                                                           | FAIL |
1 test, 0 passed, 1 failed
==============================================================================
Tmp                                                                   | FAIL |
1 test, 0 passed, 1 failed
==============================================================================
Output:  ../pabot_results/0/output.xml