PEP 672: Mention Unicode TR 55

[encukou]

Unicode Technical Report 55 “Unicode Source Code Handling” was published in January and contains lots of relevant information and general recommendations.

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This is a change to an Informational PEP, which should be OK per PEP 1 but doesn't fit the template:

• 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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