…0 spec update (python#135996)Co-authored-by: sobolevn <mail@sobolevn.me>Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>(cherry picked from commitc5e77af)

…50 spec update (#135996) (#136901)Co-authored-by: Dave Peck <davepeck@gmail.com>Co-authored-by: sobolevn <mail@sobolevn.me>

…19485)As of [this cpython PR](python/cpython#135996),it is not allowed to concatenate t-strings with non-t-strings,implicitly or explicitly. Expressions such as `"foo" t"{bar}"` are nowsyntax errors.This PR updates some AST nodes and parsing to reflect this change.The structural change is that `TStringPart` is no longer needed, since,as in the case of `BytesStringLiteral`, the only possibilities are thatwe have a single `TString` or a vector of such (representing an implicitconcatenation of t-strings). This removes a level of nesting from manyAST expressions (which is what all the snapshot changes reflect), andsimplifies some logic in the implementation of visitors, for example.The other change of note is in the parser. When we meet an implicitconcatenation of string-like literals, we now count the number oft-string literals. If these do not exhaust the total number ofimplicitly concatenated pieces, then we emit a syntax error. To recoverfrom this syntax error, we encode any t-string pieces as _invalid_string literals (which means we flag them as invalid, record theirrange, and record the value as `""`). Note that if at least one of thepieces is an f-string we prefer to parse the entire string as anf-string; otherwise we parse it as a string.This logic is exactly the same as how we currently treat`BytesStringLiteral` parsing and error recovery - and carries with itthe same pros and cons.Finally, note that I have not implemented any changes in theimplementation of the formatter. As far as I can tell, none are needed.I did change a few of the fixtures so that we are always concatenatingt-strings with t-strings.

…0 spec update (python#135996)Co-authored-by: sobolevn <mail@sobolevn.me>Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

…0 spec update (python#135996)Co-authored-by: sobolevn <mail@sobolevn.me>Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

… PEP 750 spec update (python#135996) (python#136901)Co-authored-by: Dave Peck <davepeck@gmail.com>Co-authored-by: sobolevn <mail@sobolevn.me>