Update CPython 3.8 from 3.8.0rc1 to 3.8.0 (#370)

Author: mattip

docker/build_scripts/build_env.sh

@@ -1,7 +1,8 @@
 # source me
 
 PYTHON_DOWNLOAD_URL=https://www.python.org/ftp/python
-CPYTHON_VERSIONS="2.7.16 3.4.10 3.5.7 3.6.9 3.7.4 3.8.0rc1"
+# of the form <maj>.<min>.<rev> or <maj>.<min>.<rev>rc<n>
+CPYTHON_VERSIONS="2.7.16 3.4.10 3.5.7 3.6.9 3.7.4 3.8.0"
 
 # openssl version to build, with expected sha256 hash of .tar.gz
 # archive.