Upgrade REST API spec to OpenAPI 3

[osma]

We currently specify the REST API using Swagger 2.0. This has long ago been superseded by OpenAPI 3.0, which is nowadays also well supported by the Connexion framework.

This PR updates the REST API spec for OpenAPI 3.0 and renames the directory it is stored in from swagger to openapi. I used the Swagger Editor for an initial conversion, but I had to make several fixes until I got it working. Also there are minor changes to the implementation code in rest.py that were necessary to make the new spec work.

Fixes #644

[codecov]

Codecov Report

Base: 99.54% // Head: 99.55% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b8f6b8c) compared to base (02e60ed).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #649   +/-   ##
=======================================
  Coverage   99.54%   99.55%           
=======================================
  Files          87       87           
  Lines        5968     6006   +38     
=======================================
+ Hits         5941     5979   +38     
  Misses         27       27           

Impacted Files	Coverage Δ
tests/test_rest.py	100.00% <ø> (ø)
annif/__init__.py	88.46% <100.00%> (ø)
annif/rest.py	96.36% <100.00%> (+0.13%)	⬆️
annif/parallel.py	100.00% <0.00%> (ø)
annif/registry.py	100.00% <0.00%> (ø)
tests/test_cli.py	100.00% <0.00%> (ø)
annif/backend/http.py	100.00% <0.00%> (ø)
tests/test_parallel.py	100.00% <0.00%> (ø)
tests/test_backend_http.py	100.00% <0.00%> (ø)

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[osma]

There are still problems with parameter handling

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication