Fix ensurepip segfault for 3.6, add ensurepip test

[julianneswinoga]

Fixes #23

Just needed to apply patches from https://bugs.python.org/issue45700 to 3.6

Tested:

$ nix shell ~/projects/nixpkgs-python#'"3.6.14"' --extra-experimental-features nix-command --extra-experimental-features flakes --no-eval-cache
warning: ignoring untrusted substituter 'https://nixpkgs-python.cachix.org', you are not a trusted user.
Run `man nix.conf` for more information on the `substituters` configuration option.
$ python3.6 -m venv --clear .venv
$ . ./.venv/bin/activate
$ pip3 install -U pip
Collecting pip
  Using cached https://files.pythonhosted.org/packages/a4/6d/6463d49a933f547439d6b5b98b46af8742cc03ae83543e4d7688c2420f8b/pip-21.3.1-py3-none-any.whl
Installing collected packages: pip
  Found existing installation: pip 18.1
    Uninstalling pip-18.1:
      Successfully uninstalled pip-18.1
Successfully installed pip-21.3.1

I also added an ensurepip smoke test.

CI seems to fail but for unrelated changes?

<snip>
2024-09-30T15:05:44.8522165Z v2.7.18.6_CVE.tar.gz> curl: (56) The requested URL returned error: 404
2024-09-30T15:05:44.8524611Z v2.7.18.6_CVE.tar.gz> error: cannot download v2.7.18.6_CVE.tar.gz from any mirror
2024-09-30T15:05:44.8683718Z error: builder for '/nix/store/ljxz1qjhxrml9shv3aknfjjlycx8nzmp-v2.7.18.6_CVE.tar.gz.drv' failed with exit code 1;
2024-09-30T15:05:44.8693603Z        > curl: (56) The requested URL returned error: 404
2024-09-30T15:05:44.8694508Z        > error: cannot download v2.7.18.6_CVE.tar.gz from any mirror
2024-09-30T15:05:44.8733242Z error: 1 dependencies of derivation '/nix/store/2c852bmj7hrrfy7r1nzlndmnfgqvc87l-python-2.7.18.6._CVE.drv' failed to build
<snip>
2024-09-30T15:11:16.4611034Z error: 1 dependencies of derivation '/nix/store/0pa6gzc0nry6n5mrn5aqapa9lhmbq6s1-2.7.18.6_CVE-test-ssl.drv' failed to build
2024-09-30T15:11:16.4759157Z error: build of '/nix/store/0pa6gzc0nry6n5mrn5aqapa9lhmbq6s1-2.7.18.6_CVE-test-ssl.drv', '/nix/store/2c852bmj7hrrfy7r1nzlndmnfgqvc87l-python-2.7.18.6._CVE.drv' failed