test: Remove python3.4 workaround in feature_dbcrash

Summary:
Partial backport of core PR14954 (commit fa27978):
https://github.com/bitcoin/bitcoin/pull/14954/commits/fa2797808ea0f055d1df7977deee80cc63e7b323

Depends on D4372.

Test Plan:

./test/functional/test_runner.py feature_dbcrash

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4373