bpo-38662: Invoke pip via runpy, in ensurepip

This way, any changes to the internal pip API won't mean ensurepip needs to be
changed as well. Also, distributors can update their pip wheels independent on
CPython release schedule.

Co-Authored-By: Pradyun Gedam <pradyunsg@gmail.com>
Co-Authored-By: Miro Hrončok <miro@hroncok.cz>

Author: pradyunsg

Lib/ensurepip/__init__.py

@@ -1,6 +1,7 @@
 import os
 import os.path
 import sys
+import runpy
 import tempfile
 from importlib import resources
 
@@ -26,9 +27,17 @@ def _run_pip(args, additional_paths=None):
     if additional_paths is not None:
         sys.path = additional_paths + sys.path
 
-    # Install the bundled software
-    import pip._internal
-    return pip._internal.main(args)
+    # Invoke pip as if it's the main module, and catch the exit.
+    backup_argv = sys.argv[:]
+    sys.argv[1:] = args
+    try:
+        runpy.run_module("pip", run_name="__main__", alter_sys=True)
+    except SystemExit as e:
+        return e.code
+    finally:
+        sys.argv[:] = backup_argv
+
+    raise SystemError("pip have not exited, that should never happen")
 
 
 def version():

Misc/NEWS.d/next/Library/2020-03-10-15-32-31.bpo-38662.o1DMXj.rst

@@ -0,0 +1,4 @@
+The ``ensurepip`` module now invokes ``pip`` via the ``runpy`` module.
+Hence it is no longer tightly coupled with the internal API of the bundled
+``pip`` version, allowing easier updates to a newer ``pip`` version both
+internally and for distributors.