Update python options in the generators README.md (#20417)

- Remove the dead link to a python-legacy client generator.
- Add a link to the python-pydantic-v1 client geneator.
- Add a link to the python-fastapi server generator.

docs/generators/README.md

@@ -40,7 +40,7 @@ The following generators are available:
 * [php-dt](php-dt.md)  
 * [powershell](powershell.md)  
 * [python](python.md)
-* [python-legacy](python-legacy.md)
+* [python-pydantic-v1](python-pydantic-v1.md)
 * [r](r.md)  
 * [ruby](ruby.md)  
 * [rust](rust.md)  
@@ -100,6 +100,7 @@ The following generators are available:
 * [php-symfony](php-symfony.md)  
 * [python-aiohttp](python-aiohttp.md)  
 * [python-blueplanet](python-blueplanet.md)  
+* [python-fastapi](python-fastapi.md)  
 * [python-flask](python-flask.md)  
 * [ruby-on-rails](ruby-on-rails.md)  
 * [ruby-sinatra](ruby-sinatra.md)