PEP 750: major updates to "Template Strings" based on community feedb…

…ack (#4062)

Co-authored-by: Koudai Aono <[email protected]>
Co-authored-by: pauleveritt <[email protected]>
Co-authored-by: Lysandros Nikolaou <[email protected]>
Co-authored-by: Adam Turner <[email protected]>
Co-authored-by: Nice Zombies <[email protected]>