Drop support for old versions of Flask

[gmacon]

As @nickw444 mentioned in https://github.com/nickw444/flask-ldap3-login/pull/66#discussion_r317557306, we can reduce the complexity by requiring a moderately new version of Flask. The first thing mentioned (teardown_appcontext) was added in Flask 0.9, but we may want to depend on something even newer if it gives additional benefits.

[auto-comment[bot]]

Thank you for raising an issue.

Unfortunately I have made the difficult decision that I will no longer be actively maintaining this repo. I have too much on my plate to give this library the love it needs and deserves. New features and bug fixes have became harder to manually test as I no longer had redily available testing environments, giving me lower confidence in iterating and improving the library.

I am now in the process of finding a new maintainer - if this is something that may interest you, you can find more details in the README.

[nickw444]

I will turn this off ^ 😅