ci: publish for scala3

[tkrs]

Closes #134

[codecov]

Codecov Report

Merging #184 (02ab0ed) into master (b8fe35e) will decrease coverage by 6.35%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   55.94%   49.58%   -6.36%     
==========================================
  Files           9       17       +8     
  Lines         404      486      +82     
  Branches       21       21              
==========================================
+ Hits          226      241      +15     
- Misses        178      245      +67     

Flag	Coverage Δ
unittests	49.58% <80.00%> (-6.36%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...dules/core/src/main/scala/mess/codec/Decoder.scala	85.71% <ø> (ø)
...dules/core/src/main/scala/mess/codec/Encoder.scala	82.85% <0.00%> (ø)
...rc/main/scala_2/mess/codec/DerivedEncoderOps.scala	50.00% <ø> (ø)
.../main/scala_2.12-/mess/codec/DerivedDecoder1.scala	100.00% <100.00%> (ø)
.../main/scala_2.13+/mess/codec/DerivedDecoder1.scala	100.00% <100.00%> (ø)
...rc/main/scala_2/mess/codec/DerivedDecoderOps.scala	92.85% <100.00%> (ø)
...la_2.12-/mess/internal/ScalaVersionSpecifics.scala	100.00% <0.00%> (ø)
.../main/scala_2.12-/mess/codec/DerivedEncoder1.scala	60.00% <0.00%> (ø)
...re/src/main/scala/mess/codec/DecodingFailure.scala	0.00% <0.00%> (ø)
....12/src_managed/main/mess/codec/TupleDecoder.scala	4.54% <0.00%> (ø)
... and 4 more

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