Reset file offsets before sending a request

[samamorgan]

With our request retry mechanism on HTTP 401 responses, the follow-up request that contains files to be uploaded will fail because the file has already been read by Requests to determine the file size.

This patch ensures that all files passed into a request are at offset 0 before sending the request.

[github-actions[bot]]

Coverage Report (Python 3.8) •

File	Stmts	Miss	Cover	Missing
welkin
client.py	132	9	93%	181, 225–229, 242, 276, 288
util.py	111	1	99%	142
TOTAL	1022	48	95%

Tests	Skipped	Failures	Errors	Time
230	3 :zzz:	0 :x:	0 :fire:	3.362s :stopwatch:

[github-actions[bot]]

Coverage Report (Python 3.10) •

File	Stmts	Miss	Cover	Missing
welkin
client.py	132	9	93%	181, 225–229, 242, 276, 288
util.py	111	1	99%	142
TOTAL	1022	48	95%

Tests	Skipped	Failures	Errors	Time
230	3 :zzz:	0 :x:	0 :fire:	3.431s :stopwatch:

[github-actions[bot]]

Coverage Report (Python 3.9) •

File	Stmts	Miss	Cover	Missing
welkin
client.py	132	9	93%	181, 225–229, 242, 276, 288
util.py	111	1	99%	142
TOTAL	1022	48	95%

Tests	Skipped	Failures	Errors	Time
230	3 :zzz:	0 :x:	0 :fire:	3.542s :stopwatch:

[github-actions[bot]]

Coverage Report (Python 3.11) •

File	Stmts	Miss	Cover	Missing
welkin
client.py	132	9	93%	181, 225–229, 242, 276, 288
util.py	111	1	99%	142
TOTAL	1022	48	95%

Tests	Skipped	Failures	Errors	Time
230	3 :zzz:	0 :x:	0 :fire:	3.327s :stopwatch:

[github-actions[bot]]

Coverage Report (Python 3.12) •

File	Stmts	Miss	Cover	Missing
welkin
client.py	132	9	93%	181, 225–229, 242, 276, 288
util.py	111	1	99%	142
TOTAL	1022	48	95%

Tests	Skipped	Failures	Errors	Time
230	3 :zzz:	0 :x:	0 :fire:	4.132s :stopwatch: