PEP 720: add "Cross-compiling Python packages" PEP draft (#3192)

Co-authored-by: Hugo van Kemenade <[email protected]>
Co-authored-by: Petr Viktorin <[email protected]>

Author: 3 people

.github/CODEOWNERS

@@ -597,7 +597,7 @@ pep-0713.rst  @ambv
 pep-0714.rst  @dstufft
 pep-0715.rst  @dstufft
 pep-0719.rst  @Yhg1s
-# pep-0720.rst (reserved for https://github.com/python/peps/pull/3192)
+pep-0720.rst  @FFY00
 pep-0721.rst  @encukou
 # ...
 # pep-0754.txt