Various fixes as suggested and a dependency issue I missed earlier. I had editted the wrong file.

mingw-w64-python-flit/PKGBUILD

@@ -7,6 +7,7 @@ pkgver=3.5.1
 pkgrel=1
 pkgdesc='Simplified packaging of Python modules (mingw-w64)'
 arch=('any')
+mingw_arch=('mingw32' 'mingw64' 'ucrt64' 'clang64' 'clang32' 'clangarm64')
 url='https://github.com/takluyver/flit'
 license=('BSD')
 depends=("${MINGW_PACKAGE_PREFIX}-python" 
@@ -149,6 +150,7 @@ package_python-flit-core() {
     sed -e "s|$srcdir||g" -i {} \;
 }
 
+# template start; name=mingw-w64-splitpkg-wrappers; version=1.0;
 for _name in "${pkgname[@]}"; do
   _short="package_${_name#${MINGW_PACKAGE_PREFIX}-}"
   _func="$(declare -f "${_short}")"