FIX-#3371: Remove pandas patch level pin

[mvashishtha]

What do these changes do?

Remove pandas patch level pin as we agreed here.

• first commit message and PR title follow format outlined here

  NOTE: If you edit the PR title to match this format, you need to add another commit (even if it's empty) or amend your last commit for the CI job that checks the PR title to pick up the new PR title.

• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves Do not pin pandas down to patch level #3371
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date

[YarShev]

Imagine a case when the user has pandas=1.3.5 and python 3.7. Then he wants to install modin. However, Modin only supports python>=3.8. Maybe we should remove pandas patch level pin starting 2.0?

[mvashishtha]

users on pandas 1.3.5 and python 3.7 can't use the latest version of modin anyway because modin now requires python >= 3.8. I think this is a separate issue. What do you suggest that we change?

I suspect that even importing the latest modin code on python 3.7 would break because we make so many assumptions about what we can import from pandas. That was why backwards compatibility with python 3.6 was so much work. We should only consider backwards compatibility with python 3.7 if we know of a desire for it.

[YarShev]

The changes now are fine by me.

[mvashishtha]

@YarShev @anmyachev ready for merge

[vnlitvinov]

Do note that this change is potentially opening a can of worms for us because we use some internal pandas things (be them functions, classes or variables), for which pandas is free to change or even remove those without breaking SemVer logic (as they're not public API).

But then, we can always pin version again if such thing happens...