configure travis to use py37 for docs

Parallel builds are disabled in sphinx for python
3.8. sphinx-doc/sphinx#6803

Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.travis.yml

@@ -12,7 +12,7 @@ matrix:
     - python: "nightly"
   include:
     - env: BUILD=docs
-      python: 3.8
+      python: 3.7
     - env: BUILD=linter
       python: 3.8
     - env: BUILD=pkglint