Use Python module to run Pip

setup.py used systemcalls for package management pip. This call is
platform dependent as on ubuntu distros this needs to be pip3. On these
platforms pip points to the python2 version.

By direct calling the pip module from within the running python process
we know for sure we are triggering the correct one.

Differential revision: https://developer.blender.org/D4952/

Reviewed by: sybren

setup.py

@@ -23,6 +23,7 @@ import shutil
 import subprocess
 import re
 import pathlib
+import pip
 import zipfile
 
 from distutils import log
@@ -121,9 +122,8 @@ class BuildWheels(Command):
 
     def download_wheel(self, requirement):
         """Downloads a wheel from PyPI and saves it in self.wheels_path."""
-
+        pip.main([
-        subprocess.check_call([
+            'download',
-            'pip', 'download',
             '--no-deps',
             '--dest', str(self.wheels_path),
             requirement[0]