Revert usage of subprocess.check_output text parameter

It's the same as universal_newlines, so just use the one that
works with our specified target of Python 3.5.

securedrop_client/crypto.py

@@ -153,7 +153,7 @@ def fingerprints(self) -> typing.Dict[str, bool]:
         cmd = self._gpg_cmd_base()
         cmd.extend(["--list-public-keys", "--fingerprint", "--with-colons",
                     "--fixed-list-mode", "--list-options", "no-show-photos"])
-        output = subprocess.check_output(cmd, text=True)
+        output = subprocess.check_output(cmd, universal_newlines=True)
 
         fingerprints = {}
         for line in output.splitlines():