0.30 release

While the 0.29 release is not yet out the door, there have been a few topics which have been notable but not quite ready to ship in 0.29. I wanted to list these here now so that we can avoid forgetting these as we work towards 0.30.

• list iterator issues #6087
• migrate to PyClassGuard from PyRef - #6083
  • #6120
  • #6121
• OverflowError / ValueError consistency - #6116, #5179
• build performance - #6113
• I would love to make progress on better GC support - #6330 / #5663
• meson support - #6321
• 3.15.0rc2 crash - #6390