py3: make all tests pass

[chapoton]

This ticket is used to keep track of the progress towards python3.

release	files having failing doctests	more than 100	from 10 to 100	from 1 to 10
8.4.b3	915	42	251	622
8.4.b4	803	23	175	605
8.4.b5	657	17	145	495

Folders where all tests pass (8.4.b5):

• algebras/steenrod #26219 and #26220
• arith #26208 ✔
• combinat/sf (#26220 +) #26215
• features ✔
• finance #26213 ✔
• games ✔
• gsl
• knots ✔
• lfunctions ✔
• logic #26214 ✔
• media ✔
• parallel ✔
• probability #26216 ✔
• quadratic_forms/ #26224 ✔
• quivers #26211 ✔
• server ✔
• typeset ✔

Folders with work in progress:

• data_structures #24852
• docs #26310
• plot #26210 (partial fix only) #26299
• matrix/ #26236 (very partial fix)
• modules #26276
• monoids #26294
• rings/number_field #26263 (partial fix only)
• tensor #26295
• interfaces #26314
• numerical/backends #24740 (for char * arguments management)
• sandpiles #26016