Added ability to insert delay between page requests and corrected pub_year to year

[stanleyrhodes]

Fixes: https://github.com/scholarly-python-package/scholarly/issues/431

Description

Scholarly did not seem to have the ability to reduce the rate of page requests. Tools such as Publish or Perish limit this rate to prevent GS lockout, so this seemed a reasonable option to add. I added the ability to have it be a single number of seconds, or to pass a custom function that generated the number in seconds. The two could be combined, if one wanted to have a constant plus a little random padding from a function.

In addition, bibtex generation seemed to mistakenly produce a field called pub_year that needed to be year to meet proper bibtex format (.e.g., Zotero would not pick up the year with pub_year being the year field, but it does as year). There was no obvious benefit to this field being pub_year, and evidence a previous contributor meant for it to be year so I changed it to year in all cases.

Checklist

• [x] Check that the base branch is set to develop and not main.
• [ ] Ensure that the documentation will be consistent with the code upon merging. (Should we document the delay feature?)
• [ ] Add a line or a few lines that check the new features added. (I'm not sure how to do this.)
• [x] Ensure that unit tests pass. (python -m unittest test_module.TestScraperAPI passes) If you don't have a premium proxy, some of the tests will be skipped. The tests that are run should pass without raising MaxTriesExceededException or other exceptions.

[arunkannawadi]

Merging this to a new branch so I could clean up the commits and merged to develop