PEP 735: Add include-project to Deferred Ideas

[sirosen]

This is a fairly simple addition -- writing up an idea which surfaced
in the discourse thread as Deferred. The rationale given here, which
is the primary one for deferral, is that this would be obviated by
(and therefore create a confusing situation in combination with) Path
Dependencies.

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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