added python flask sample

[HugoMario]

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 and ./bin/security/{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\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

added flask sample.

[wobeng]

@HugoMario does this PR allow code generation for open 3.0 for python?

[HugoMario]

Hi @wobeng , yes it does !

[wobeng]

@HugoMario how can I use it?

[HugoMario]

python and python flask are currently on SNAPSHOT version, you can grab it latest bbuild here

and to gennerate code locally you can use this command (as example):

java -jar ./java -jar ./swagger-codegen-cli-3.0.9-20190610.160559-25.jar generate -i http:https://petstore.swagger.io/v2/swagger.json -l python-flask -o output

[HugoMario]

@wobeng

[wobeng]

@HugoMario you are the best, will try this