Avoid --system-site-packages during the snap build by preparing a venv with pipstrap that already includes wheel (#8445)

This PR proposes an alternative configuration for the snap build that avoid the need to use `--system-site-package` when constructing the virtual environment in the snap.

The rationale of `--system-site-package` was that by default, snapcraft creates a virtual environment without `wheel` installed in it. However we need it to build the wheels like `cryptography` on ARM architectures. Sadly there is not way to instruct snapcraft to install some build dependencies in the virtual environment before it kicks in the build phase itself, without overriding that entire phase (which is possible with `parts.override-build`).

The alternative proposed here is to not override the entire build part, but just add some preparatory steps that will be done before the main actions handled by the `python` snap plugin. To do so, I take advantage of the `--upgrade` flag available for the `venv` module in Python 3. This allows to reuse a preexisting virtual environment, and upgrade its component. Adding a flag to the `venv` call is possible in snapcraft, thanks to the `SNAPCRAFT_PYTHON_VENV_ARGS` environment variable (and it is already used to set the `--system-site-package`).

Given `SNAPCRAFT_PYTHON_VENV_ARGS` set to `--upgrade` , we configure the build phase as follows:
* create the virtual environment ourselves in the expected place (`SNAPCRAFT_PART_INSTALL`)
* leverage `tools/pipstrap.py` to install `setuptools`, `pip`, and of course, `wheel`
* let the standard build operations kick in with a call to `snapcraftctl build`: at that point the `--upgrade` flag will be appended to the standard virtual environment creation, reusing our crafted venv instead of creating a new one.

This approach has also the advantage to invoke `pipstrap.py` as it is done for the other deployable artifacts, and for the PR validations, reducing risks of shifts between the various deployment methods.

snap/snapcraft.yaml

@@ -64,7 +64,6 @@ parts:
       - libpython3.8-stdlib
       - libpython3.8-minimal
       - python3-pip
-      - python3-setuptools
       - python3-wheel
       - python3-venv
       - python3-minimal
@@ -74,13 +73,16 @@ parts:
     # To build cryptography and cffi if needed
     build-packages: [gcc, libffi-dev, libssl-dev, git, libaugeas-dev, python3-dev]
     build-environment:
-      - SNAPCRAFT_PYTHON_VENV_ARGS: --system-site-packages
+      - SNAPCRAFT_PYTHON_VENV_ARGS: --upgrade
       - PIP_NO_BUILD_ISOLATION: "no"
+    override-build: |
+      python3 -m venv "${SNAPCRAFT_PART_INSTALL}"
+      "${SNAPCRAFT_PART_INSTALL}"/bin/python3 "${SNAPCRAFT_PART_SRC}/tools/pipstrap.py"
+      snapcraftctl build
     override-pull: |
-        snapcraftctl pull
-        cd $SNAPCRAFT_PART_SRC
-        python3 tools/strip_hashes.py letsencrypt-auto-source/pieces/dependency-requirements.txt | grep -v python-augeas > snap-constraints.txt
-        snapcraftctl set-version `grep -oP "__version__ = '\K.*(?=')" $SNAPCRAFT_PART_SRC/certbot/certbot/__init__.py`
+      snapcraftctl pull
+      python3 "${SNAPCRAFT_PART_SRC}/tools/strip_hashes.py" "${SNAPCRAFT_PART_SRC}/letsencrypt-auto-source/pieces/dependency-requirements.txt" | grep -v python-augeas > "${SNAPCRAFT_PART_SRC}/snap-constraints.txt"
+      snapcraftctl set-version `grep -oP "__version__ = '\K.*(?=')" "${SNAPCRAFT_PART_SRC}/certbot/certbot/__init__.py"`
   shared-metadata:
     plugin: dump
     source: .