Pin mypy to 0.630.

Pin mypy to 0.630 to work around errors reported in mypy 0.641.

.travis.yml

@@ -23,7 +23,9 @@ addons:
         packages:
             - python3-pip
 install:
-    - pip3 install --user --upgrade mypy flake8
+    # Fix the mypy version at 0.630 since version 0.641 is more strict and the
+    # code is not yet updated.
+    - pip3 install --user --upgrade mypy==0.630 flake8
     - mypy --version
 before_script:
     # If an old version of rustfmt from cargo is already installed, uninstall