Fix CloudFront signed URL unit tests for Python 3.
Encode RSA private key strings as UTF-8 before writing them to binary
temporary files. Python 3 requires bytes when writing to files opened
in binary mode.
--- ../tests/unit/cloudfront/test_signed_urls.py	2026-10-18 01:48:34.296036908 +0300
+++ ../tests/unit/cloudfront/test_signed_urls.py	2026-10-18 01:49:10.415267004 +0300
@@ -107,7 +107,7 @@
                     "Nx6FucDB7OVqzcxkxHsGFd8VCG1BkC-Afh9~lOCMIYHIaiOB6~5j"
                     "t9w2EOwi6sIIqrg_")
         pk_file = tempfile.TemporaryFile()
-        pk_file.write(self.pk_str)
+        pk_file.write(self.pk_str.encode('utf-8'))
         pk_file.seek(0)
         sig = self.dist._sign_string(self.canned_policy, private_key_file=pk_file)
         encoded_sig = self.dist._url_base64_encode(sig)
@@ -123,7 +123,7 @@
                     "Nx6FucDB7OVqzcxkxHsGFd8VCG1BkC-Afh9~lOCMIYHIaiOB6~5j"
                     "t9w2EOwi6sIIqrg_")
         pk_file = tempfile.NamedTemporaryFile()
-        pk_file.write(self.pk_str)
+        pk_file.write(self.pk_str.encode('utf-8'))
         pk_file.flush()
         sig = self.dist._sign_string(self.canned_policy, private_key_file=pk_file.name)
         encoded_sig = self.dist._url_base64_encode(sig)