Disable mypy cache

Mypy crashes with "AssertionError: Cannot find module for UpdatedBase"
when analyzing lnt/server/db/migrations/upgrade_0_to_1.py with a
populated cache. This leads to a number of files not being analyzed by
mypy.

Reviewed By: cmatthews

Differential Revision: https://reviews.llvm.org/D95033

Author: Thomas Preud'homme

tox.ini

@@ -21,9 +21,13 @@ deps =
     sqlalchemy-stubs
     future
 commands =
-    # No where close to passing yet, but nice to have.
-    - mypy --install-types --non-interactive --junit-xml=junit-{envname}.xml --py2 --ignore-missing-imports lnt
-    - mypy --install-types --non-interactive --junit-xml=junit-{envname}.xml --ignore-missing-imports lnt
+    # Nowhere close to passing yet, but nice to have. The option
+    # --no-incremental is currently needed to suppress warnings about
+    # UpdatedBase when running tests several times. Future versions of
+    # mypy should be checked to see if it can be removed and not get
+    # warnings after running tox several times.
+    - mypy --no-incremental --junit-xml=junit-{envname}.xml --py2 --ignore-missing-imports lnt
+    - mypy --no-incremental --junit-xml=junit-{envname}.xml --ignore-missing-imports lnt
 
 [testenv:flake8]
 skip_install = true