Patch new pip to make it support the old private API of main()

This is needed to be able to use pip 19.3+ in ensurepip of older Pythons
without patching all the Pythons.

python-pip.spec

@@ -97,6 +97,15 @@ Patch4:         dummy-certifi.patch
 Patch5:         network-tests.patch
 %endif
 
+# Dark magic to make pip 19.3+ support the old private API of main()
+# This is needed to be able to use pip 19.3+ in ensurepip of older Pythons
+# without patching all the Pythons
+# A proper fix is discussed in:
+#   https://discuss.python.org/t/can-we-finally-add-a-minimal-api-to-pip/2833/3
+#   https://github.com/pypa/pip/pull/7061
+#   https://github.com/python/cpython/pull/16782
+Patch6:         callable-main.patch
+
 # Downstream only patch
 # Users might have local installations of pip from using
 # `pip install --user --upgrade pip` on older/newer versions.
@@ -254,6 +263,7 @@ popd
 %if %{with tests}
 %patch5 -p1
 %endif
+%patch6 -p1
 
 # this goes together with patch4
 rm src/pip/_vendor/certifi/*.pem