Don't cast list to tuple in python-prior binding

[Ichimonji10]

Tweak the python-prior API bindings, so that they no longer cast lists to tuples when making a POST request with a multipart/form-data content-type. This fixes an interaction with
urllib3.request_encode_body, whose fields parameter expects tuples, not lists.

cc @spacether

See: https://urllib3.readthedocs.io/en/stable/reference/urllib3.request.html

Fix: #14012

[Ichimonji10]

I tested with the following:

mvn clean package
sudo update-alternatives --set java java-11-openjdk.x86_64
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh

mvn clean install
java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
  -i ~/Downloads/sample.yaml \
  -g python-prior \
  -o ~/Downloads/out/

For info on the call to update-alternatives, see #13684.

For info on the generation from sample.yaml, see #14012.

I have also tested this change against an internal service at my company.

[spacether]

Hey there. This fix looks great but it has no verification test.
Can you please add a test in the tests_manual directory?
If that test is omitted, then this code could be changed in the future, breaking this feature and we would not see any error indicating that something had broken.

[Ichimonji10]

Ack, will do.

[Ichimonji10]

WIP, please ignore for now. I will ping when ready for review.

Note to self:

mvn integration-test -f samples/openapi3/client/petstore/python-prior/pom.xml
cd samples/openapi3/client/petstore/python-prior/
./test_python.sh

[Ichimonji10]

Done. This MR now contains two commits, the first of which adds a test, case, and the second of which adds a code fix:

git checkout urllib3~1
./test_python.sh  # fail

git checkout urllib3
./test_python.sh  # success

EDIT: Rebased so that some comments could be tweaked.

[spacether]

This looks great, thank you for the fix and adding the test!