Address SonarCloud complaint: use strings in __all__ statement

[osma]

This PR addresses a number of similar complaints by SonarCloud: the __all__ list defined in __init__.py should consists of strings.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #454 (503641a) into master (846ebaf) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #454   +/-   ##
=======================================
  Coverage   99.41%   99.41%           
=======================================
  Files          65       65           
  Lines        4625     4625           
=======================================
  Hits         4598     4598           
  Misses         27       27           

Impacted Files	Coverage Δ
annif/corpus/__init__.py	100.00% <100.00%> (ø)

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