CI: Add a CI job for testing using conda

[Smit-create]

Fixes #514

cc @mmcky

[mmcky]

thanks @Smit-create this is looking great -- really appreciate it.

I have approved for GitHub actions to run this PR.

[coveralls]

Coverage remained the same at 94.451% when pulling c71b12b on Smit-create:conda_ci into 5be42da on QuantEcon:master.

[Smit-create]

this is looking great

Thanks! This seems to be working fine. We can also use conda environment cache for faster CI (not that effective in case of mac) but we can do it as the follow up of this PR.

[mmcky]

Thanks @Smit-create -- the Conda cache idea is also great 👍 - thanks for your contribution.

[mmcky]

• We might also deprecate the old testing