Check whether GPG is actually available rather than inferring from OS (#1439)

Cargo.lock

@@ -35,9 +35,9 @@ checksum = "b248f5224d1d606005e02c97f5aa4e88eeb230488bcc03bc9ca4d7991399f2b5"
 
 [[package]]
 name = "libc"
-version = "0.2.162"
+version = "0.2.164"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "18d287de67fe55fd7e1581fe933d965a5a9477b38e949cfa9f8574ef01506398"
+checksum = "433bfe06b8c75da9b2e3fbea6e5329ff87748f0b144ef75306e674c3f6f7c13f"
 
 [[package]]
 name = "memchr"

tests/compat/test_porcelain.py

@@ -21,8 +21,6 @@
 """Compatibility tests for dulwich.porcelain."""
 
 import os
-import platform
-import sys
 from unittest import skipIf
 
 from dulwich import porcelain
@@ -31,15 +29,17 @@ from dulwich.tests.utils import build_commit_graph
 from ..test_porcelain import PorcelainGpgTestCase
 from .utils import CompatTestCase, run_git_or_fail
 
+try:
+    import gpgme
+except ImportError:
+    gpgme = None
+
 
 @skipIf(
-    platform.python_implementation() == "PyPy" or sys.platform == "win32",
-    "gpgme not easily available or supported on Windows and PyPy",
+    gpgme is None,
+    "gpgme not available, skipping tests that require GPG signing",
 )
 class TagCreateSignTestCase(PorcelainGpgTestCase, CompatTestCase):
-    def setUp(self):
-        super().setUp()
-
     def test_sign(self):
         # Test that dulwich signatures can be verified by CGit
         c1, c2, c3 = build_commit_graph(