feat: pass a filename to oauth()

[bastienboutonnet]

Aim

While load_credentials() allows for user's custom credentials path for service account credentials, oauth() doesn't but as discussed in #826 it looks like it is something several users would appreciate.

Given that oauth() simply wraps around load_credentials() it seems pretty straightforward to just allow oauth() to recieve an optional filename.

Solution

• oauth() gets a an oprional credentials_filename argument which is then passed down to load_credentials() and defaults to DEFAULT_AUTHORIZED_USER_FILENAME so that the change is backward compatible with current documented usage.
• Adds documentation snippets
• Closes Allow gspread.oauth() to look for file in another directory #826

To Reviewer's Attention

• I named the argument credentials_filename, however, I see that you folks generally just use filename, happy to keep things consistent here
• I also have black turned on by default on my IDE which resulted in a formatting change across the entire file. I imagine that's not entirely welcome in this project so I'm happy to go ahead and correct that back. Unless you'd like to use black across your project and I'd be happy to help with that.
• I couldn't find a unit test on the function I modified but let me know if I'm missing something

[burnash]

Thank you very much for the pull request.

Please revert to the original formatting as it makes the review much easier.

[bastienboutonnet]

@burnash Thanks for reviewing and sorry for the delay in replying. I've changed back the formatting, sorry about that and made some of the changes you requested. I'm not sure about this comment though: #847 (comment)

And also, I wrote an answer to this one: #847 (comment) explaining the reason for my change.

Let me know if there's anything else that needs to change.

[bastienboutonnet]

@burnash thanks for the additional pointers and good catches. I believe I've implemented the changes you requested.

[lavigne958]

Hi @bastienboutonnet I checked your PR, looks good to me ! 👍

Could you please squash your commits in order to have only a single commit that introduces the feature ? If you don't feel confident with this kind of git tricks tell me I'll guide you through it.

Then we could merge this relevant feature into master branch 😉

[bastienboutonnet]

Hi @lavigne958 thanks a lot for your review, I've just squashed so feel free to merge.

Thanks for asking. I generally just squash and merge contributor's branches on my OS project but this is a nice touch :)

[lavigne958]

Thank you @bastienboutonnet for your help, right I could do it this way. or cherry-pick every commits or... so many ways to do it with git 😆

[bastienboutonnet]

The pleasure was mine @lavigne958