Fix openapi3 request parse validation

[Lounarok]

This is a fix for #357.
It's a problem caused by extracting schemas in a location which is changed by openapi3.

This is a PR depends on #504 but it's not very hard to merge without it.
Only last two commits are related to this PR

Fix request parse for openapi3. 8e731d5
Add tests for parse openapi3 request 5d16c30

Please let me know your decision.

[Lounarok]

I got the same issues like #503.
Some test build are failed in pip intsall with weird errors..

Processing dependencies for flasgger-package==0.0.1
error: zipp 0.5.1 is installed but zipp>=3.1.0; python_version < "3.10" is required by {'importlib-resources'}
Makefile:11: recipe for target 'flasgger_package' failed
make: *** [flasgger_package] Error 1
The command "make test" exited with 2.

[Lounarok]

@billyrrr
I think previous CI build failed is because of pip 20.3's bug.
You may compare the success test and the failed test.

I found the difference is pip version and it's why my PR #503 failed while only ReadMe.md is modified.
I added #503's changes into this branch. Please let me know if you think they should be separate into two different PRs.

[billyrrr]

Thank you for these improvements. I have merged the code without a review. Some modifications and fixes may be made to these features before release over pypi.

[Lounarok]

Hi @billyrrr, thanks for the merge. About the fixes or modifications, do you find something need to fix immediately or refer to some potential bugs?
If it's the existing bugs, please let me know the issue number.
If it's the latter, I'll help for fixing it.

[mcclayton]

HI @billyrrr and @Lounarok, I'd love to see this make its way to a published release as I am currently trying to get our swagger to be OpenAPI 3.0 compliant, and would love to be able to use @Lounarok's changes. Thank you both for your contributions!

[Lounarok]

@mcclayton I had this issue as well.
I did a pypi for this change until new official version is released.

[mcclayton]

Thanks @Lounarok 🎉
I'll use this for testing, but would still love to be able to pin to an official release once one is published.

[nijave]

requirements.txt

git+https://github.com/flasgger/flasgger.git@beb9fa781fc6b063fe3f3081b9677dd70184a2da#egg=flasgger

For convenience :)