Add proxy support for pypi package managers (#150)

* initial pypi registry implementation

* support proxy mode for pypi package managers

* support proxy mode for pypi package managers - 2

* rm default mode as proxy for pip3

* update goproxy version & fix pypi proxy failing on 304

* add PIP_RETRIES=0 env

* update pmg e2e & add proxy mode e2e for pypi

* rm safedep-test-pkg for pypi proxy e2e

docs/proxy-mode.md

@@ -31,6 +31,6 @@ proxy_mode: true
 | `pnpx`          | ✅      |
 | `bun`           | ✅      |
 | `yarn`          | ✅      |
-| `pip`           | 🕒      |
-| `uv`            | 🕒      |
-| `poetry`        | 🕒      |
+| `pip`           | ✅      |
+| `uv`            | ✅      |
+| `poetry`        | ✅      |