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 +1,43 @@
+#!/usr/bin/env python3
+
+import argparse
+import subprocess
+import sys
+from pathlib import Path
+
+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()
+
+    print(">>> Getting PyMuPDF deps as requirements.txt", file=sys.stderr)
+    cmd = ["poetry", "export", "--only", "container"]
+    container_requirements_txt = subprocess.check_output(cmd)
+
+    print(f">>> Vendoring PyMuPDF under '{args.dest}'", file=sys.stderr)
+    cmd = [
+        "python3",
+        "-m",
+        "pip",
+        "install",
+        "--no-cache-dir",
+        "--no-compile",
+        "-t",
+        args.dest,
+        "-r",
+        "/proc/self/fd/0",  # XXX: pip does not read requirements.txt from stdin
+    ]
+    subprocess.check_output(cmd, input=container_requirements_txt)
+
+    print(f">>> Successfully vendored PyMuPDF under '{args.dest}'", file=sys.stderr)
+
+
+if __name__ == "__main__":
+    sys.exit(main())