setup: relax -Werror for declaration-after-statement on Python 3.9 It turns out Python 3.9 introduces such declarations in the headers, eg cpython/abstract.h:189:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement] so we have to be more relaxed when compiling for 3.9. Differential Revision: https://phab.mercurial-scm.org/D8327