Remove -Wconversion from CFLAGS_NODIST in --enable-safety

[nohlson]

Bug report

Bug description:

-Wconversion in CFLAGS_NODIST in CI jobs Ubuntu build/test and macOS build/test emits warnings on PRs that have changes unrelated to those warnings. There is an issue in the actions toolkit expressing the same frustration: actions/toolkit#457

We also need to discuss how we will address remediating -Wconversion related warnings if we decide to enable it again.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-124191: Disable -Wconversion in enable safety #124192