Orphan hash needs an alpha char

Fixes twisted#584

src/towncrier/create.py

@@ -83,11 +83,15 @@ def __main(
 
     file_dir, file_basename = os.path.split(filename)
     if config.orphan_prefix and file_basename.startswith(f"{config.orphan_prefix}."):
-        # Append a random hex string to the orphan news fragment base name.
+        # Append a random hex string (with at least one alphabetic character to avoid
+        # clashing with issue numbers) to the orphan news fragment base name.
+        random_hash = ""
+        while not random_hash or random_hash.isdigit():
+            random_hash = os.urandom(4).hex()
         filename = os.path.join(
             file_dir,
             (
-                f"{config.orphan_prefix}{os.urandom(4).hex()}"
+                f"{config.orphan_prefix}{random_hash}"
                 f"{file_basename[len(config.orphan_prefix):]}"
             ),
         )

src/towncrier/test/test_create.py

@@ -3,7 +3,6 @@
 
 import os
 import string
-
 from pathlib import Path
 from textwrap import dedent
 from unittest import mock
@@ -229,6 +228,27 @@ def test_create_orphan_fragment(self, runner: CliRunner):
         # Length should be '+' character and 8 random hex characters.
         self.assertEqual(len(change2.stem), 9)
 
+    def test_create_orphan_fragment_must_contain_alpha(self):
+        """
+        The random value added to the orphan fragment base name must contain at least
+        one alphabetic character to avoid clashing with issue numbers.
+        """
+        setup_simple_project()
+
+        frag_path = Path("foo", "newsfragments")
+
+        with mock.patch("towncrier.create.os.urandom") as mock_urandom:
+            mock_urandom(4).hex.side_effect = [
+                "12345678",
+                "56781234",
+                "123a5678",
+                "abcdefgh",
+            ]
+            result = CliRunner().invoke(_main, ["+.feature"])
+        self.assertEqual(0, result.exit_code, result.output)
+        fragments = list(frag_path.rglob("*"))
+        self.assertEqual(fragments, [frag_path / "+123a5678.feature"])
+
     @with_isolated_runner
     def test_create_orphan_fragment_custom_prefix(self, runner: CliRunner):
         """