Add in PDF link in publication fill

[Luen]

Hi team,

I noticed that Scholarly doesn't output the PDF link (only pub_url), so I'm attempting to add this feature.

Description

Edited PublicationParser() fill() in publication_parser.py to add HTML parsing for the pdf link.

Example publication with pdf link: https://scholar.google.com/citations?view_op=view_citation&hl=en&user=ynWS968AAAAJ&citation_for_view=ynWS968AAAAJ:8xutWZnSdmoC

Checklist

• Check that the base branch is set to develop and not main.
• Ensure that the documentation will be consistent with the code upon merging.
• Add a line or a few lines that check the new features added.
• Ensure that unit tests pass.
  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.

[TuTouPower]

Good job, bro. Use "eprint_url" not "pdf_url" will more suit.

[arunkannawadi]

Thank you for this PR and for expanding the unit tests to test this.