Bump flask from 0.12.3 to 1.0 in /example

Bumps [flask](https://github.com/pallets/flask) from 0.12.3 to 1.0.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/master/CHANGES.rst)
- [Commits](pallets/flask@0.12.3...1.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

example/requirements.txt

@@ -1,3 +1,3 @@
-flask==0.12.3
+flask==1.0
 sanic==0.7.0
 Quart==0.6.7