Add some tracemalloc hooks

Use PYTHONTRACEMALLOC=25 as envvar to enable tracemalloc on start

And then:

export SWH_CONFIG_FILENAME=/etc/softwareheritage/loader_git.yml
export PYTHONTRACEMALLOC=25
swh --log-level DEBUG loader run git <origin-url>

Don't forget the log-level flag otherwise, you won't see anything.

Related to #3625 (closed)

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Migrated from D6419 (view on Phabricator)