PEP 501: update to template strings proposal (#3047)

* Re-open PEP 501 in consideration of PEP 701
* Switch naming from "interpolation template strings" to "template literal string"
* Add Nick Humrich as co-author

---------

Co-authored-by: C.A.M. Gerlach <[email protected]>
Co-authored-by: Nick Coghlan <[email protected]>
Co-authored-by: Hugo van Kemenade <[email protected]>