tolerate pip 19.3 for --pyprof builds

[hartb]

The 'main' function was moved between pip 19.2.3 and 19.3, which
causes '--pyprof' build to fail with (at least) pip 19.3 and 19.3.1:

Traceback (most recent call last):
  File "setup.py", line 37, in <module>
    pipmain(["install"] + required_packages)
TypeError: 'module' object is not callable

A permanent solution for this might require rewriting the '--pyprof'
install section to use 'python -m pip', but this change just adjusts
the import to tolerate the 19.3 change.

See:

• pypa/pip@09fd200
• Define all old pip entrypoints pypa/pip#7498
• Add old pip entrypoints pypa/pip#7499

[hartb]

See #680

[mcarilli]

Should be fixed now via #652, I thought we merged that already.