[BUG] python-aiohttp test failure

[nhomble]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

$ git clone https://github.com/OpenAPITools/openapi-generator
$ cd openapi-generator/samples/server/petstore/python-aiohttp; mvn verify

And we find the error

./test_python3.sh: line 30: deactivate: command not found
make: *** [Makefile:18: test-all] Error 127
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  33.560 s
[INFO] Finished at: 2020-04-13T09:57:04-04:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (test) on project PythonAiohttpServer: Command execution failed.: Process exited with an error: 2 (Exit value: 2) -> [Help 1]

Suggest a fix

The .venv isn't created even though the Makefile knows how to clean. I suggest we just add it as a rule.

[nhomble]

Same issue happens with the other python-aiohttp server test.