venv: Add Flask subdependency upper bound limit.

Currently Flask v1 depends on Jinja v2, which doesn't have an upper
bound limit for MarkupSafe, and the latest release of MarkupSafe v2.1
is not compatible with Jinja v2. So we need to add the upperbound
here.

pallets/jinja#1585

Author: carlosperate

mu/wheels/__init__.py

@@ -38,6 +38,9 @@ class WheelsBuildError(WheelsError):
 #
 mode_packages = [
     ("pgzero", "pgzero>=1.2.1"),
+    # Flask v1 depends on Jinja v2, which doesn't have an upper bound limit in
+    # MarkupSafe, and v2.1 is not compatible with Jinja v2
+    ("markupsafe", "markupsafe<2.1"),
     ("flask", "flask==1.1.4"),
     # The version of ipykernel here should match to the version used by
     # qtconsole at the version specified in setup.py