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.

callable-main.patch

@@ -0,0 +1,28 @@
+diff --git a/src/pip/_internal/__init__.py b/src/pip/_internal/__init__.py
+index 8c0e4c58..76e280e5 100755
+--- a/src/pip/_internal/__init__.py
++++ b/src/pip/_internal/__init__.py
+@@ -1,2 +1,3 @@
+ #!/usr/bin/env python
+ import pip._internal.utils.inject_securetransport  # noqa
++from pip._internal import main  # noqa
+diff --git a/src/pip/_internal/main.py b/src/pip/_internal/main.py
+index 1e922402..e52e04d8 100644
+--- a/src/pip/_internal/main.py
++++ b/src/pip/_internal/main.py
+@@ -45,3 +45,15 @@ def main(args=None):
+     command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
+ 
+     return command.main(cmd_args)
++
++
++# Dark magic to make the main module itself callable.
++# This is needed to be able to use this pip in ensurepip of older Pythons
++# without patching all the Pythons.
++
++class _CallableModule(sys.modules[__name__].__class__):
++    def __call__(self, *args, **kwargs):
++        return main(*args, **kwargs)
++
++
++sys.modules[__name__].__class__ = _CallableModule

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