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This reverts commit 24de8690.

This broke at least mutter cross builds [1]. Seems to be something to do
with how shared libraries are found by our gir wrapper.

[1] https://build.voidlinux.org/builders/armv6l_builder/builds/34575/steps/shell_3/logs/stdio

Closes: #30787

Move to python_version=3. Using py2 was ok, since the lttng-gen-tp
script supported both, but it's supported py3 since [1].

[1] https://github.com/lttng/lttng-ust/commit/2d9c7df199d251c3094cd9ce241a59e78bc4ade3

Closes: #31190 [via git-merge-pr]