Introducing new optional parameter at_index to PdfWriter.addpage

[Lucas-C]

This is a copy of pmaupin#216

[pep8speaks]

Hello @Lucas-C! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-01 13:38:39 UTC

[sarnold]

Hi there, I got really busy with RL stuff but I finally did get back to this... Anyway, the static_pdfs repo for tests is now a submodule and the imports in the test files should be changed to from tests import static_pdfs|expected except after I did that, your test fails the hash assert. I don't have a bunch of free time but if you have the time to make the changes in your PRs we can get these merged. Also, please rebase against master first...

[sarnold]

Hmm, after rebase and fixing the imports, this also needed updated hashes for the test to pass. I can go ahead and merge this one and apply the updates for more testing, but you'll need to rebase/update the other 2 PRs and push them again. Thanks!

[sarnold]

Hmm, the hashes in test_add_page appear to be machine/env-specific so for now I'm going to revert my "fix" commit and disable the test for now (in tox.ini). I'm not sure what the proper fix is, and I can't really plumb the depths of PDF files/digests right now.