Bump Flask due to https://github.com/pytest-dev/pytest/issues/5532

[wbrgss]

Pins Flask in requires-install.txt to v1.0.4 in order to avoid pytest-dev/pytest#5532 (using assert in tests is broken). I suppose this could be in requires-testing.txt but I don't think it's too risky. Latest version is v1.1.2 (https://flask.palletsprojects.com/en/1.1.x/changelog/)

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • task 1
  • task 2
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follow
  • this github #PR number updates the dash docs
  • here is the show and tell thread in plotly dash community

[alexcjohnson]

@wbrgss this seems like a good idea, though unlikely to have practical consequences for most people at this point. 💃