Update flask requirement from <2.1,>=1.0 to >=1.0,<3.1

Updates the requirements on [flask](https://github.com/pallets/flask) to permit the latest version.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@1.0...3.0.0)

---
updated-dependencies:
- dependency-name: flask
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

setup.py

@@ -12,7 +12,7 @@
         "markupsafe<2.0"
     ],
     extras_require={
-        "Flask": ["Flask>=1.0,<2.1"],
+        "Flask": ["Flask>=1.0,<3.1"],
         "dev": [
             "pytest",
             "pytest-flakes",