[WIP] Move all CI/CD to Github Actions

[srmsoumya]

This PR adds support to lint, format & test the codebase across different python & gdal versions.

Description

We can run different steps as: tox -e [lint/test/format/flake8]

We are not relying on tox.ini file for running different python environments. That is taken care of using github runners. I commented out the format step as that would modify the codebase.

Fixes #440 [WIP]

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected - these changes will not be merged until major releases!)

How Has This Been Tested?

There is technically no change in the codebase, so I have not added any new testcases.

Checklist:

• [x] My PR has a descriptive title
• [ ] My code follows PEP8
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new errors
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] My PR passes Travis CI tests
• [x] My PR does not reduce coverage in Codecov

cc @rbavery @vincentsarago

[rbavery]

@rodrigoalmeida94 @srmsoumya thanks for the heads up, just added you both as admins

[rodrigoalmeida94]

@srmsoumya one thing I noticed with flake8 is the check occurs, and there are 217 errors, but the CI is not failing - trying to figure out why that is.

Also the lint command tox -e lint is only formatting the code (using black and isort, like format), I'll make a branch rewriting that step as lint. #450

[srmsoumya]

@rodrigoalmeida94 I guess you already figured this out. FWIW, adding runs like a list in tox files returns success code even when it fails the CI check.

[rodrigoalmeida94]

@srmsoumya seems the issues has to do with writing commands with -. i.e. this:

[testenv:lint]
basepython = python3
skip_install = true
deps = flake8
commands = 
    - flake8 .

Will always return success (even if there are linter errors).

[srmsoumya]

451 supersedes this PR