Create a new temporary directory for GNUPGHOME

Storing GNUPGHOME inside the build tree may result in the following when
calling gpg.Context().create_key():

gpg.errors.GPGMEError: gpgme_op_createkey: GnuPG: No agent running

This happens if the path to the build tree is so long, that the path to
the socket `S.gpg-agent` inside the GNUPGHOME exceeds 108 characters.
The limit comes from sockaddr_un.sun_path, see unix(7) or:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=847206

Long build paths are commonly found when building bmaptool inside sbuild
or on Debian gitlab CI.

By setting GNUPGHOME to a new temporary directory, which will likely be
placed in /tmp, we avoid problems with long paths to GNUPGHOME.

tests/test_CLI.py

@@ -33,8 +33,8 @@ class Key:
 
 
 testkeys = {
-    "correct": Key("tests/test-data/gnupg", "correct <foo@bar.org>"),
-    "unknown": Key("tests/test-data/gnupg2", "unknown <blub@bla.net>"),
+    "correct": Key(None, "correct <foo@bar.org>", None),
+    "unknown": Key(None, "unknown <blub@bla.net>", None),
 }
 
 
@@ -149,9 +149,8 @@ def setUp(self):
 
         os.makedirs("tests/test-data/signatures", exist_ok=True)
         for key in testkeys.values():
-            if os.path.exists(key.gnupghome):
-                shutil.rmtree(key.gnupghome)
-            os.makedirs(key.gnupghome)
+            assert key.gnupghome is None
+            key.gnupghome = tempfile.mkdtemp(prefix="bmaptool")
             context = gpg.Context(home_dir=key.gnupghome)
             dmkey = context.create_key(
                 key.uid,
@@ -194,6 +193,7 @@ def tearDown(self):
         for key in testkeys.values():
             shutil.rmtree(key.gnupghome)
             os.unlink(f"{key.gnupghome}.keyring")
+            key.gnupghome = None
             for bmapv in ["2.0", "1.4"]:
                 testp = "tests/test-data"
                 imbn = "test.image.bmap.v"