Simplify logic in PythonConfig.InstructionsForPhase()

The diff looks big, but the logical changes are small and were done in
this order:

1. Check right away if version or requirements are not specified.
   Those two are required to activate the Python builder. Old commit
   messages indicate that this was always the intended behavior.[0]

2. In code after the initial check, replaced all "pc.Requirements != nil"
   with "true" since we know that it is definitely true.

3. Removed any resulting 'if true' or 'if true || ...' expressions and
   de-indented their bodies.

This makes it much easier to understand the flow through this method,
especially the pc.usePoetry() conditionals.

[0] This did affect a test case, which I updated. I believe the new
behavior is more correct.

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Related MRs:

• !83 (merged)
• !82 (merged)
• !80 (merged) (This MR)
• main