Function annotations were added as part of Python 3.0, with PEP 3107, proposed in 2006 [0]. The first public mypy commit was in 2012 [1], and originally it was using C++-style syntax (`list<Token> lex(str s):`). The `typing` module and the official use of function annotation for type hints came in 2014 with PEP 484 [2], inspired by mypy. The first pyright commit was in 2019 (with a lot of code in the third commit [3], possibly moved from the VSCode extension).
Python typecheckers predate in-language annotations and drove the spec, not vice versa.