tests fail after 2024-06-12

[bmwiedemann]

While working on reproducible builds for openSUSE, I found that
our bmap-tools-3.6 package fails to build after 2024-06-12.

To reproduce, you could set the system clock or use the following on Debian or openSUSE:

osc co openSUSE:Factory/bmap-tools && cd $_
osc build --clean --vm-type=kvm --noservice --build-opt=--vm-custom-opt="-rtc base=2024-06-13T00:00:00" standard

actual results:

tests/test_CLI.py::TestCLI::test_valid_signature FAILED
...

----------------------------- Captured stderr call -----------------------------
bmaptool: �[91mERROR�[0m: An error occurred, here is the traceback:
Traceback (most recent call last):
   File "/home/abuild/rpmbuild/BUILDROOT/bmap-tools-3.6-0.x86_64/usr/lib/python3.8/site-packages/bmaptools/CLI.py", line 205, in verify_detached_bmap_signature
    sigs = context.verify(signed_data, signature, None)[1].signatures
  File "/usr/lib64/python3.8/site-packages/gpg/core.py", line 558, in verify
    raise errors.BadSignatures(results[1], results=results)

bmaptool: �[91mERROR�[0m: discovered a BAD GPG signature: tests/test-data/signatures/test.image.bmap.v2.0.valid-sig

This is likely due to some expiring gpg key.