simplify pypyr.steps.py syntax

[yaythomas]

working with context in a pypyr.steps.py step means working with the dict object rather than its contents:

    - name: pypyr.steps.py
      description: runs arb python
      in:
        # multi-line statement starts with |, per yaml spec
        pycode: |
                  print(f"py step: {0+1}")
                  context['arbvalue'] = 6
                  print(context['arbvalue'])

Add py mode of execution where pipeline authors don't need to trip over the context['arbkey'] syntax and can just use arbkey:

    - name: pypyr.steps.contextsetf
      comment: set arb context to manipulate in the next step
      in:
        contextSetf:
          existing_key: existing value
          existing_dict:
            a: a value
            b: 123

    - name: pypyr.steps.py
      description: runs arb python
      in:
        # multi-line statement starts with |, per yaml spec
        py: |
          from math import sqrt
          print(f"py step: {1+1}")
          
          arbvalue = sqrt(1764)
          print(int(arbvalue))

          existing_key = 'updated value'
          existing_dict['a'] = 'a value set in py step'
          existing_dict['b'] = 456

Make this backwards compatible so pipeline author can use either pycode or py inputs. Additionally, there might be occasions where access to the dict object itself is useful, rather than just its contents.

[yaythomas]

Related #205. See adr 0001-namespace-on-eval-and-exec