install: Add script for vendoring PyMuPDF

Add a script that installs PyMuPDF under ./dangerzone/vendor. This will
be useful in subsequent commits, for vendoring PyMuPDF when building
Debian packages.

install/linux/vendor-pymupdf.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+
+import argparse
+import logging
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+DZ_VENDOR_DIR = Path("./dangerzone/vendor")
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "--dest",
+        default=DZ_VENDOR_DIR,
+        help="The destination directory for the vendored packages (default: ./dangerzone/vendor)",
+    )
+    args = parser.parse_args()
+
+    logging.basicConfig(
+        level=logging.DEBUG,
+        format="%(asctime)s - %(levelname)s - %(message)s",
+        datefmt="%Y-%m-%d %H:%M:%S",
+    )
+
+    logger.info("Getting PyMuPDF deps as requirements.txt")
+    cmd = ["poetry", "export", "--only", "container"]
+    container_requirements_txt = subprocess.check_output(cmd)
+
+    # XXX: Hack for Ubuntu Focal.
+    #
+    # The `requirements.txt` file is generated from our `pyproject.toml` file, and thus
+    # specifies that the minimum Python version is 3.9. This was to accommodate to
+    # PySide6, which is installed in macOS / Windows via `poetry` and works with Python
+    # 3.9+. [1]
+    #
+    # The Python version in Ubuntu Focal though is 3.8. This generally was not much of
+    # an issue, since we used the package manager to install dependencies. However, it
+    # becomes an issue when we want to vendor the PyMuPDF package, using `pip`. In order
+    # to sidestep this virtual limitation, we can just change the Python version in the
+    # generated `requirements.txt` file in Ubuntu Focal from 3.9 to 3.8.
+    #
+    # [1] https://github.com/freedomofpress/dangerzone/pull/818
+    if sys.version.startswith("3.8"):
+        container_requirements_txt = container_requirements_txt.replace(b"3.9", b"3.8")
+
+    logger.info(f"Vendoring PyMuPDF under '{args.dest}'")
+    # We prefer to call the CLI version of `pip`, instead of importing it directly, as
+    # instructed here:
+    # https://pip.pypa.io/en/latest/user_guide/#using-pip-from-your-program
+    cmd = [
+        sys.executable,
+        "-m",
+        "pip",
+        "install",
+        "--no-cache-dir",
+        "--no-compile",
+        "--target",
+        args.dest,
+        "--requirement",
+        "/proc/self/fd/0",  # XXX: pip does not read requirements.txt from stdin
+    ]
+    subprocess.run(cmd, check=True, input=container_requirements_txt)
+
+    if not os.listdir(args.dest):
+        logger.error(f"Failed to vendor PyMuPDF under '{args.dest}'")
+
+    logger.info(f"Successfully vendored PyMuPDF under '{args.dest}'")
+
+
+if __name__ == "__main__":
+    sys.exit(main())