Upgrade Flask version to 3.0.0.

[TunaCici]

The problem specified in issue #15 is caused by outdated dependencies. Upgrading Flask to newer version fixes it.

This PR fixes #15 by upgrading the Flask (and its) dependencies to their latest available versions. Tested on:

• Ubuntu 22.04 LTS (virtual Python 3.9)
• macOS 14.2.1 (virtual Python 3.11.6)

Upgrading dependencies fixes not only the dependency problem, but also some critical vulnerabilities found on Flask versions 2.2.5 and older.

More information about Flask vulnerabilities can be found on SYNK - Flask.

Here's this PR running flask-example on a Raspberry Pi 4b (home server):

[TunaCici]

Just to mention that this PR ups the Python version requirement to be 3.8 due to Flask dropping support for 3.7.

See the Flask 2.3.0 changes.