Revert "[data] Fix pandas import failures by moving it to a top-level data import"

[cadedaniel]

Reverts #32447

Looks like #32447 breaks bk://:mac: :apple: Wheels and Jars and linux://python/ray/tests:test_basic

Failed BK: https://buildkite.com/ray-project/oss-ci-build-branch/builds/2327#01864d8e-0ebd-4ef3-bfda-1259dfcfcbbc

+ /Library/Frameworks/Python.framework/Versions/3.6/bin/python3.6 /Users/ec2-user/.buildkite-agent/builds/bk-mac1-branch-queue-i-0b5fcfa84a3210bb0-1/ray-project/oss-ci-build-branch/ci/build/../../python/ray/tests/test_microbenchmarks.py

Traceback (most recent call last):
  File "/Users/ec2-user/.buildkite-agent/builds/bk-mac1-branch-queue-i-0b5fcfa84a3210bb0-1/ray-project/oss-ci-build-branch/ci/build/../../python/ray/tests/test_microbenchmarks.py", line 5, in <module>
    import ray
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/ray/__init__.py", line 257, in <module>
    from ray import data  # noqa: F401
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/ray/data/__init__.py", line 3, in <module>
    import pandas  # noqa
ModuleNotFoundError: No module named 'pandas'

[clarkzinzow]

@ericl I would have mentioned this, didn't get a chance to review the PR, but the ray.data import is still eager for all ray imports for Python 3.6:

ray/python/ray/__init__.py

Lines 255 to 258 in d87d86f

	if sys.version_info < (3, 7):
	# TODO(Clark): Remove this one we drop Python 3.6 support.
	from ray import data # noqa: F401
	from ray import workflow # noqa: F401

[clarkzinzow]

We can backport a module-level __getattr__ for Python 3.6 with something like this: #27658 (comment), or we can try to search for a different solution, like fixing the underlying Core-level issue.

Also, are we ever going to drop Python 3.6? We're now over a year past the EOL, with 4 months until the Python 3.7 EOL. cc @zhe-thoughts @pcmoritz

[cadedaniel]

merge please? cc @clarkzinzow @jianoaix @ericl