Make test database cache lookup concurrent has_cache() is used to dertermine if the template database already exists or not. When running tests accross multiple process, has_cache() wasn't aware of databases created by other processes because it used a cache to generate the cache... Since the discovering discover_cached_db() time is quite reasonable (it's just about listing file in a directory), let re-generate the cache on cache miss.

Fix flake8 issues since release 3.6.0 Flake8 had a new release which raise new issues, namely: W504: line break after binary operator F841: local variable 'ex' is assigned to but never used W605: invalid escape sequence F821: undefined name 'buffer' (noqa seems the only way to avoid this false positive) Also pin flake8>=3.6 in our tests and make explicit that we use python3 to run flake8 tests.

Stabilize flake8 tests on 3.26 branch Use basepython=python2 or flake8 installed with python3 will raise "[F821] undefined name 'buffer'" Pin flake8 < 3.6 which has new checks that doesn't pass on our current code base.