gh-159560: Add a command-line interface to tracemalloc - #159563

Why make this change

Today, to check memory allocations of a script with tracemalloc you have to modify the script itself. This adds python -m tracemalloc, so it can be used the same way as cProfile, without touching the code. Discussed in https://discuss.python.org/t/run-tracemalloc-module-as-a-script/50821.

$ python -m tracemalloc -l 5 script.py
Top 5 lineno allocations still alive at exit:
script.py:2: size=981 KiB, count=101, average=9942 B
<frozen runpy>:266: size=434 B, count=4, average=108 B
<frozen runpy>:102: size=400 B, count=1, average=400 B
<frozen runpy>:81: size=400 B, count=1, average=400 B
script.py:1: size=168 B, count=1, average=168 B
2 other: size=90 B, count=2
Total size: 982 KiB
Peak size: 983 KiB

What will change

• Add _main() to Lib/tracemalloc.py with options -o, -t, --cumulative, -n, -l and -m
• Run the target with runpy, so sys.argv, __name__ and sys.path[0] are the same as when it runs directly. Directories and zip files work too
• Keep the globals of the traced program alive until the snapshot is taken, otherwise module level data is freed before the report
• Strip the oldest runpy / tracemalloc frames from the tracebacks, so --cumulative and -t traceback show only the frames of the traced program
• Restart tracing if it was already enabled by -X tracemalloc or PYTHONTRACEMALLOC, otherwise -n is silently ignored
• With -o, dumped snapshots use tracemalloc.Snapshot ( not __main__.Snapshot ), so they can be loaded with Snapshot.load()
• Add docs, a What's New entry and tests

Known limitations, same as cProfile:

• a missing -m module raises ImportError with a traceback. Checking it before tracing would import its parent packages untraced
• a -- right after the target is removed by argparse ( same in profiling.sampling )

• Issue: Add a command-line interface to tracemalloc #159560