[Python][server] Use range for dependencies in python-flask

Created by: GuillaumeSmaha

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first. Scripts called:

./bin/python-server-flask-petstore.sh
./bin/python-server-flask-petstore-python2.sh
./bin/openapi3/python-flask-petstore.sh
./bin/openapi3/python-flask-petstore-python2.sh

• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @Jyhess (2019/01)

Description of the PR

The aim is to use >= instead of == to avoid error when the generated server is used directly as a package dependency.