Fwdpy11

* Refactor mutation metadata definition for compatibility with tskit 0.6.1
* Fix pinning of rust toolchain version
* Clean up unused Python imports

No new features or bug fixes.

This version focuses on improving reproducible builds and dependency management.

Bug fix:

* Raise error when simulating neutral mutations and
tracking mutation counts suppresses table indexing.
(PR {pr}`1342`)

Removes an unintended `print` statement from the C++ back end code.

Bug fixes:

Fix bug in handling fixation removal in generations
when simplification occurs. (PR {pr}`1326`)

Back end changes:

* Improve runtime validation of inputs to GSL discrete
lookup tables. (PR {pr}`1330`)
* Throw an exception if ALL diploid fitness values are 0. (PR {pr}`1330`)
* Throw an exception if ALL diploid fitness values are 0 within a single deme. (PR {pr}`1330`)
* Fix a warning relating to code not accepted in C++20. (PR {pr}`1333`)

Bug fixes:

Fix bug in handling fixation removal (1323)

* In effect, flags to remove fixations were ignored.
This likely occurred when modifying the back end
to support demes graphs, but I have not bothered to
bisect out the source of the error.
* Added explicit C++ and Python tests of removing fixations
* Updated Python tests of selective sweeps, including
fixing some logic errors in the tests that led them
to pass "by luck" rather than by design.

This bug does not affect simulation results for multiplicative
models. For such models, relative fitness is what matters
and not removing fixations still maintains relative fitness.