Use tailored user-agent in requests by HTTP-backend

[juhoinkinen]

The http-backend uses requests library, and so the headers have now user-agent python-requests/2.28.1, which is shown e.g. in logs of the targeted server.

It is better to use a user-agent tailored for Annif. This PR changes the user-agent that http-backend sends in request headers to Annif/<version>, e.g. currently to Annif/0.60.0.dev0.

[codecov]

Codecov Report

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

Coverage data is based on head (27f1bb1) compared to base (02e60ed).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #645   +/-   ##
=======================================
  Coverage   99.54%   99.55%           
=======================================
  Files          87       87           
  Lines        5968     6004   +36     
=======================================
+ Hits         5941     5977   +36     
  Misses         27       27           

Impacted Files	Coverage Δ
annif/backend/http.py	100.00% <100.00%> (ø)
tests/test_backend_http.py	100.00% <100.00%> (ø)
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)
tests/test_parallel.py	100.00% <0.00%> (ø)

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[osma]

LGTM!