PEP 1 and 12: Remove optional Content-Type from templates

[hugovk]

Fixes #2500.

• There are three 👍s on Is Content-Type optional? #2500 (comment), for removing the optional Content-Type header from the PEP 1 and 12 templates: @JelleZijlstra, @encukou, @hugovk, @CAM-Gerlach.

• @AA-Turner was -0 in Is Content-Type optional? #2500 (comment)

But following issue #10 / PR #3418, all PEPs now have an .rst extension, so we no longer need the optional header for new PEPs.

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📚 Documentation preview 📚: https://pep-previews--3454.org.readthedocs.build/

[AA-Turner]

As it happens I have a draft branch that removes Content-Type from all PEPs. But I thought likely too disruptive given the spate of recent changes; this is a nice compromise. Thanks!

A