Problem: pip==21.1 support broken

Solution: change imports appropriately

src/zc/buildout/easy_install.py

@@ -40,6 +40,7 @@ import zc.buildout
 import zc.buildout.rmtree
 from zc.buildout import WINDOWS
 from zc.buildout import PY3
+from zc.buildout.pip_support import pip_install_cmd as _pip_install_cmd
 import warnings
 import csv
 
@@ -182,7 +183,6 @@ def _execute_permission():
     return 0o777 - current_umask
 
 
-_pip_install_cmd = 'from pip.__main__ import _main; _main()'
 
 def get_namespace_package_paths(dist):
     """

src/zc/buildout/pip_support.py

+try: 
+    # support pip>=19.0
+    from pip._internal.cli.main import main
+    pip_install_cmd = 'from pip._internal.cli.main import main; main()'
+except ImportError:
+    # support pip>=10.0
+    from pip._internal import main
+    pip_install_cmd = 'from pip._internal import main; main()'