Running the PEG Parser benchmarks requires having memory_profiler installed.

In Tools/peg_generator:

➜  peg_generator git:(master) make time_stdlib
../../python -m zipfile -e data/xxl.zip data
../../python scripts/benchmark.py --parser=cpython --target=xxl compile
Traceback (most recent call last):
  File "/home/lysnikolaou/repos/cpython/Tools/peg_generator/scripts/benchmark.py", line 9, in <module>
    import memory_profiler
ModuleNotFoundError: No module named 'memory_profiler'

I propose to make that optional and only compute the timing benchmarks, in case memory_profiler is not available, since it's these benchmarks that are most important and most frequently run.