Add support for aarch64 python 3.9 wheels (#123)

Author: janaknat

.travis.yml

@@ -32,6 +32,11 @@ jobs:
         - MB_PYTHON_VERSION=3.8
         - NP_BUILD_DEP="numpy==1.19.1"
         - NP_TEST_DEP="numpy==1.19.1"
+    - os: linux
+      env:
+        - MB_PYTHON_VERSION=3.9
+        - NP_BUILD_DEP="numpy==1.19.1"
+        - NP_TEST_DEP="numpy==1.19.1"
 before_install:
     # See:
     # https://github.com/travis-ci/travis-ci/issues/8920#issuecomment-352661024