Added tox configuration and updating dependency reqs

[IndexSeek]

Updating requirements and adding tox.

The current repo is using a Dockerfile to test Python 3.9 compatibility. I'm proposing using tox to automatically execute tests across different versions of Python.

There have been some inconsistencies recently with the cryptography and pyOpenSSL libraries between versions and I'm hoping this will ensure we can safely test the package installs and unit tests on each version of Python that schemachange supports.

Closes #165

[IndexSeek]

I updated the setup and requirements to support a looser restriction around the required snowflake-connector-python package.

After this change and running tox each environment successfully installed and tests passed.

.pkg: _exit> python venv\lib\site-packages\pyproject_api\_backend.py True setuptools.build_meta
  lint: OK (5.67=setup[4.47]+cmd[1.20] seconds)
  type: OK (5.25=setup[3.66]+cmd[1.59] seconds)
  py37: OK (30.99=setup[28.97]+cmd[2.02] seconds)
  py38: OK (4.73=setup[3.64]+cmd[1.09] seconds)
  py39: OK (4.64=setup[3.56]+cmd[1.08] seconds)
  py310: OK (4.76=setup[3.64]+cmd[1.12] seconds)
  congratulations :) (56.16 seconds)

[fozcodes]

Any chance this can be merged? This is preventing us from using the newest connector in a lot of projects.

[svitty]

@sfc-gh-jhansen

[ryan-keswick]

pls, my dependabot is crying

[harringj]

@sfc-gh-jhansen, do you have any sense of when this PR might get merged? i noticed that no commits have occurred since Feb 15 of this year. should we assume this project will no longer be maintained? thank you!

[sfc-gh-jhansen]

Included in the new 3.5.3 release. Thanks!