Add PYTHON_BUILD_TCLTK_FORMULA to allow building against Tcl/Tk 9

Tcl/Tk 9.0 support was added in 3.13.0b3 and is not official as of 3.14.0;
the build with Homebrew version fails as of this writing

plugins/python-build/README.md

@@ -175,6 +175,7 @@ You can set certain environment variables to control the build process.
 * `PYTHON_BUILD_SKIP_HOMEBREW`, if set, will not search for libraries installed by Homebrew when it would normally will.
 * `PYTHON_BUILD_USE_HOMEBREW`, if set, will search for libraries installed by Homebrew when it would normally not.
 * `PYTHON_BUILD_HOMEBREW_OPENSSL_FORMULA`, override the Homebrew OpenSSL formula to use.
+* `PYTHON_BUILD_TCLTK_FORMULA`, override the Homebrew Tcl/Tk formula to use.
 * `PYTHON_BUILD_SKIP_MACPORTS`, if set, will not search for libraries installed by MacPorts when it would normally will.
 * `PYTHON_BUILD_USE_MACPORTS`, if set, will search for libraries installed by MacPorts when it would normally not.
 * `PYTHON_BUILD_ROOT` overrides the default location from where build definitions

plugins/python-build/bin/python-build

@@ -1884,7 +1884,7 @@ use_homebrew_tcltk() {
   # tcl-tk is 9.0 which is not compatible with CPython as of this writing
   # but we'll keep it as backup for cases like non-updated Homebrew
   local tcltk
-  for tcltk in "tcl-tk@8" "tcl-tk"; do
+  for tcltk in ${PYTHON_BUILD_TCLTK_FORMULA:-tcl-tk@8 tcl-tk}; do
     local tcltk_libdir="$(brew --prefix "${tcltk}" 2>/dev/null || true)"
     if [ -d "$tcltk_libdir" ]; then
       echo "python-build: use tcl-tk from homebrew"

plugins/python-build/test/build.bats

@@ -46,7 +46,7 @@ tarball() {
 #!$BASH
 echo "$name: CFLAGS=\\"\$CFLAGS\\" CPPFLAGS=\\"\$CPPFLAGS\\" LDFLAGS=\\"\$LDFLAGS\\" PKG_CONFIG_PATH=\\"\$PKG_CONFIG_PATH\\"" >> build.log
 echo "$name:" "\$@" \${PYTHONOPT:+PYTHONOPT=\$PYTHONOPT} >> build.log
-${extra_vars:+echo \"$name: $extra_vars\" >>build.log}
+${extra_vars:+echo $name: $extra_vars >>build.log}
 OUT
 
   for file; do
@@ -794,6 +794,40 @@ make install
 OUT
 }
 
+@test "tcl-tk is linked from Homebrew with PYTHON_BUILD_TCLTK_FORMULA" {
+  cached_tarball "Python-3.6.2"
+  tcl_tk_version=8.6
+  tcl_tk_libdir="$BATS_TEST_TMPDIR/homebrew-tcl-tk"
+  mkdir -p "$tcl_tk_libdir/lib"
+  echo "TCL_VERSION='$tcl_tk_version'" >>"$tcl_tk_libdir/lib/tclConfig.sh"
+
+  stub uname '-s : echo Darwin'
+  stub sw_vers '-productVersion : echo 1010'
+
+  stub brew "--prefix tcl-tk-custom : echo '$tcl_tk_libdir'"
+  for i in {1..4}; do stub brew false; done
+
+  stub_make_install
+
+  run_inline_definition <<DEF
+PYTHON_BUILD_TCLTK_FORMULA=tcl-tk-custom
+install_package "Python-3.6.2" "http://python.org/ftp/python/3.6.2/Python-3.6.2.tar.gz"
+DEF
+  assert_success
+
+  unstub uname
+  unstub sw_vers
+  unstub brew
+  unstub make
+
+  assert_build_log <<OUT
+Python-3.6.2: CFLAGS="" CPPFLAGS="-I${BATS_TEST_TMPDIR}/install/include" LDFLAGS="-L${BATS_TEST_TMPDIR}/install/lib -Wl,-rpath,${BATS_TEST_TMPDIR}/install/lib" PKG_CONFIG_PATH="${BATS_TEST_TMPDIR}/homebrew-tcl-tk/lib/pkgconfig"
+Python-3.6.2: --prefix=${BATS_TEST_TMPDIR}/install --enable-shared --libdir=${BATS_TEST_TMPDIR}/install/lib --with-tcltk-includes=-I${BATS_TEST_TMPDIR}/homebrew-tcl-tk/include --with-tcltk-libs=-L${BATS_TEST_TMPDIR}/homebrew-tcl-tk/lib -ltcl$tcl_tk_version -ltk$tcl_tk_version
+make -j 2
+make install
+OUT
+}
+
 @test "tcl-tk is linked from Homebrew via pkg-config and override vars" {
   cached_tarball "Python-3.6.2" - TCLTK_CFLAGS TCLTK_LIBS
   tcl_tk_version=8.6