Bump Ray Version from 2.0.0.dev0 to 3.0.0.dev0

[simon-mo]

To prepare for 2.x series of release. The master dev version should
always be higher than any released version number.

Why are these changes needed?

This is part of changes to bump major version for Ray 2.0.

Essentially a massive find and replace for

• 2.0.0.dev0 -> 3.0.0.dev0
• 2.0.0dev0 -> 2.0.0dev0

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[simon-mo]

runtime_env tests failing ok

def test_get_master_wheel_url():
        ray_version = "3.0.0.dev0"
        test_commit = "58a73821fbfefbf53a19b6c7ffd71e70ccf258c7"
        for sys_platform in ["darwin", "linux", "win32"]:
            for py_version in ["36", "37", "38", "39"]:
                url = get_master_wheel_url(
                    test_commit, sys_platform, ray_version, py_version
                )
>               assert requests.head(url).status_code == 200, url
E               AssertionError: https://s3-us-west-2.amazonaws.com/ray-wheels/master/58a73821fbfefbf53a19b6c7ffd71e70ccf258c7/ray-3.0.0.dev0-cp36-cp36m-macosx_10_15_intel.whl
E               assert 404 == 200
E                 +404
E                 -200

Linkcheck failing OK with the same reason.

Other tests flaky