tidy ResilientSession implementation

[adehad]

Summary of Changes

• Resilient Session code cleanup, removing dead code and other code repetition
• Updated workaround for retrying the MultipartEncoder scheme for attachments

TODO

• Probably broke some tests that need to be fixed
• Make sure docstrings are ok
• add some tests for the new retry object

Special Thanks:

@datashaman : https://stackoverflow.com/questions/23267409/how-to-implement-retry-mechanism-into-python-requests-library/23271433#comment58969710_23271433

Co-Authored-By: datashaman <59514+datashaman@users.noreply.github.com>

[adehad]

@StefanoBoriero , could I bother you to have a look at this?

As you were recently in this code I thought you might be able to critique this !

[StefanoBoriero]

@StefanoBoriero , could I bother you to have a look at this?

As you were recently in this code I thought you might be able to critique this

With pleasure! I'll find some time in the next few days to do so

[StefanoBoriero]

Good improvements! I left some comments mostly about refactoring, let me know what you think

[adehad]

Thanks @StefanoBoriero, I've tried to address most of the concerns, with some discussions points.

I've organised my commits to tackle individual comments (as GitHub a bit weird this way) and left it up to you to resolve threads that I've addressed.

[studioj]

quite a lengthy PR ... I'd like to test this out before giving my "official" approval :) I'll try to do so this week

[studioj]

rework looks ok and the code is covered in the functional tests ... only the extra doc still todo

[adehad]

Thanks @studioj , I'll make some time to finish the PR