Enable py37 base-builds (bob.conda)

This MR does the first step in enabling python 3.7 builds for the core infrastructure (base packages, i.e. those in bob.conda will now be built in both py36 and py37 variants).

bob/devtools/data/gitlab-ci/base-build.yaml

@@ -24,6 +24,7 @@ stages:
     - conda activate base
     - bdt ci base-build -vv nopython.txt
     - bdt ci base-build -vv --python=3.6 python.txt
+    - bdt ci base-build -vv --python=3.7 python.txt
     - '[ "${CI_COMMIT_REF_NAME}" = "master" ] && bdt ci base-deploy -vv'
     - bdt ci clean -vv
   cache: &build_caches