Meta-ticket: Use new features from FLINT 2.x

[mkoeppe]

As of Sage 9.4, Sage ships FLINT 2.7.1 (#31069), accepts system FLINT >= 2.6.0, and is ready for FLINT 2.8.x.

Many of the features added in the FLINT 2.x series are so far unused in Sage. See ​https://github.com/wbhart/flint2/blob/trunk/NEWS (newest items at the end)

Tickets/features:

• #29894 minimal interface for using ZZ[x] matrices in Flint
• #31525 Wrap FLINT 2.6 functions, drop support for system FLINT < 2.6
• #32175 Support FLINT 2.8.0
• #31548 Use FLINT's nmod_mat_t for matrices mod n.
• #31666 very fast multi-variate polynomials - use them for Sage polynomial rings and perhaps in sage.tensor.modules

For creating the bindings to the new FLINT functions, there is a potential for synergy with ​https://github.com/fredrik-johansson/python-flint; the SymPy? developers have ​announced (2021) that they plan to use python-flint.

[slelievre]

Possibly related: #16812, #16664, #16429.

Older: #9541, #8939.

[mkoeppe]

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.