test_sigchain_fileobj test fails when GnuPG >= 2.2.8

Relevant change in GnuPG 2.2.8: https://dev.gnupg.org/T3981

Test output:-
======================================================================
ERROR: test_sigchain_fileobj (testing.unit.test_collections.CollectionTest)
Test getting signature chain fileobjs from archive_dir
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/testing/unit/test_collections.py", line 188, in test_sigchain_fileobj
    self.sigchain_fileobj_check_list(self.sigchain_fileobj_get(None))
  File "/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/testing/unit/test_collections.py", line 180, in sigchain_fileobj_check_list
    test_fileobj(0, "Hello, world!")
  File "/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/testing/unit/test_collections.py", line 177, in test_fileobj
    fileobjlist[i].close()
  File "/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/duplicity/dup_temp.py", line 227, in close
    assert not self.fileobj.close()
  File "/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/duplicity/gpg.py", line 304, in close
    self.gpg_failed()
  File "/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/duplicity/gpg.py", line 271, in gpg_failed
    raise GPGError(msg)
GPGError: GPG Failed, see log below:
===== Begin GnuPG log =====
gpg: starting migration from earlier GnuPG versions
gpg: porting secret keys from '/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/testing/gnupg/secring.gpg' to gpg-agent
gpg: key 839E6A2856538CCF: secret key imported
gpg: key BCEF1B4DB5FA894F: secret key imported
gpg: key 453005CE9B736B2A: secret key imported
gpg: migration succeeded
gpg: CAST5 encrypted data
gpg: encrypted with 1 passphrase
gpg: WARNING: message was not integrity protected
gpg: Hint: If this message was created before the year 2003 it is
likely that this message is legitimate. This is because back
then integrity protection was not widely used.
gpg: Use the option '--ignore-mdc-error' to decrypt anyway.
gpg: decryption forced to fail!
gpg: WARNING: unsafe permissions on homedir '/tmp/guix-build-duplicity-0.7.17.drv-0/duplicity-0.7.17/testing/gnupg'
===== End GnuPG log =====