tests: collapse some more trivial if PYTHON3 blocks
This doesn't account for all of the references to PYTHON3. But it
accounts for the ones that are more trivial and don't entail logical
changes.
Differential Revision: https://phab.mercurial-scm.org/D12235
# Rust builds with a modern MSVC and uses a newer CRT.
# Python 2.7 has a shared library dependency on an older CRT (msvcr90.dll).
# We statically link the modern CRT to avoid multiple msvcr*.dll libraries
# being loaded and Python possibly picking up symbols from the newer runtime
# (which would be loaded first).
[target.'cfg(target_os = "windows")']
rustflags = ["-Ctarget-feature=+crt-static"]