Allow pex to be invoked using runpy (python -m pex).

[jaraco]

Today I went to run pex using pip-run, but found that running python -m pex results in the usual error:

python3: No module named pex.__main__; 'pex' is a package and cannot be directly executed

Of course, one can use python -m pex.bin.pex but that's unintuitive. How about let the pex package also supply the pex command?

[jsirois]

Thanks @jaraco and please accept my apologies for the significant delay looking at this. I've fixed up the isort failure and if this goes green I'll merge.