CLN: Deprecate dayofweek/hello day_of_week (#9606)
Rename dayofweek properties in Period, Datetime, and PeriodArray classes.
Add day_of_week property to respective tests for each class.

- [x] closes #9606
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@arw2019 Thank you for the review. I addressed your comments and added an entry to whatsnew file. @arw2019 Resolved minor issues. Do you know how to locally test .rst files? In the CI stage of the checks, I am failing things like
`/home/runner/work/pandas/pandas/pandas/core/indexes/period.py:docstring of pandas.PeriodIndex.rst:77: WARNING: autosummary: stub file not found 'pandas.PeriodIndex.day_of_year'. Check your autosummary_generate setting.
2391`
AND
`/home/runner/work/pandas/pandas/doc/source/whatsnew/v1.2.0.rst:31: WARNING: Explicit markup ends without a blank line; unexpected unindent.`

`./ci/code_checks.sh` shows no errors locally > @arw2019 Resolved minor issues. Do you know how to locally test .rst files? In the CI stage of the checks, I am failing things like
> `/home/runner/work/pandas/pandas/pandas/core/indexes/period.py:docstring of pandas.PeriodIndex.rst:77: WARNING: autosummary: stub file not found 'pandas.PeriodIndex.day_of_year'. Check your autosummary_generate setting. 2391`

I think you (maybe) need to add the new methods to the rst files manually - for example in doc/source/reference/series.rst both    daysinmonth and days_in_month are listed

> `/home/runner/work/pandas/pandas/doc/source/whatsnew/v1.2.0.rst:31: WARNING: Explicit markup ends without a blank line; unexpected unindent.`

This one is a linting issue
 @arw2019 The only unit test that is failing on Azure pipeline is `test_binary_arith_ops` which seems to be a known unreliable test (#37328). (The test suite passes on my macOS machine)

Is this good to merge now? @pandas-dev/pandas-core should we do a DeprecationWarning / FutureWarning on the original attributes? (would be a followon) @jreback All checks green except aforementioned `test_binary_arith_ops` thanks @abhishekmangla very nice