unbound: use the brewed Python (#16728)

When the python alias was removed from the python Formula, unbound
reverted to using the system Python, but unboundmodule.py is installed
for the brewed python. That broke the Python integration.

Use the configure env var PYTHON_VERSION to make Unbound find the brewed
python2.7 binary.

Formula/unbound.rb

@@ -26,6 +26,7 @@ class Unbound < Formula
 
     if build.with? "python"
       ENV.prepend "LDFLAGS", `python-config --ldflags`.chomp
+      ENV.prepend "PYTHON_VERSION", "2.7"
 
       args << "--with-pyunbound"
       args << "--with-pythonmodule"