Ubuntu 20.04: migrate the codebase from 16.04 to 20.04 Ubuntu version.

[Toktar]

Migrate Indy Plenum codebase to Ubuntu20.04 and new Python.

Acceptance criteria:

• Bump Python version from 3.6 to 3.8
• Bump all dependencies where it's possible without risky code changes
• All existing tests should pass

[Toktar]

Codebase was updated in this PR https://github.com/hyperledger/indy-plenum/pull/1522 But new dependencies versions was strictly pinned. PR for remove pins where possible: https://github.com/hyperledger/indy-plenum/pull/1534

In scope of issue https://github.com/hyperledger/indy-plenum/issues/1536 new failed tests were found:

• testSimpleSigningWithSimpleKeys - fixed via pinning libnacl==1.6.1 here https://github.com/hyperledger/indy-plenum/pull/1534
• test_new_view_combinations - fails because of issue https://github.com/hyperledger/indy-plenum/issues/1506. The test was skipped in the master brunch and merged to ubuntu-20.04-upgrade in PR https://github.com/hyperledger/indy-plenum/pull/1532

[Toktar]

PR https://github.com/hyperledger/indy-plenum/pull/1534 - merged PR https://github.com/hyperledger/indy-plenum/pull/1532 - merged

[WadeBarnes]

Done