[core change!] replace bsddb3 with default Python dbm

[pawelmhm]

Reasons.

1. bsddb3 is deprecated after Python 3.5
2. successor to this library is difficult to install on many systems
3. dbm is Python standard library, should work on all platforms

Effect:

We can support all Python versions between 3.5 ad 3.9, tests pass on versions of Python > 3.5.

TODO

• run benchmark to see how dbm compares with bsddb3 in terms of performance

[pawelmhm]

After running benchmarks there is no degradation in performance. Actually dbm is better than berkeley DB. I used berkeleydb python package for testing, which claims to continue bsddb3, I had to build both python berkeley and OS berkeleyDB from source on llinux, ubuntu 20.04

benchmark file is in repo in tests directory for everyone to reproduce and for future tests. Median time for berkeley db 120 ms on master branch, median dfor dbm 94 ms.

Branch results:

Master results (with berkeleydb):