Prepare develop for release v0.5.1

[DavidMinarsch]

Proposed changes

This PR bumps the aea version and aligns all package versions.

Fixes

Types of changes

What types of changes does your code introduce to agents-aea?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have checked that code coverage does not decrease.
• I have checked that the documentation about the aea cli tool works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

[codecov-commenter]

Codecov Report

Merging #1494 into develop will increase coverage by 0.00%.
The diff coverage is 71.42%.

@@           Coverage Diff            @@
##           develop    #1494   +/-   ##
========================================
  Coverage    89.92%   89.92%           
========================================
  Files          243      243           
  Lines        17237    17239    +2     
========================================
+ Hits         15501    15503    +2     
  Misses        1736     1736           

Flag	Coverage Δ
#unittests	89.92% <71.42%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
aea/test_tools/test_cases.py	84.87% <33.33%> (+0.08%)	⬆️
aea/__version__.py	100.00% <100.00%> (ø)
packages/fetchai/connections/ledger/base.py	71.18% <100.00%> (ø)
packages/fetchai/connections/oef/connection.py	65.25% <100.00%> (ø)
...kages/fetchai/connections/p2p_libp2p/connection.py	85.59% <100.00%> (ø)
packages/fetchai/connections/soef/connection.py	100.00% <100.00%> (ø)
packages/fetchai/connections/tcp/base.py	100.00% <100.00%> (ø)

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