Fixup #2196

[joschmitt]

I realized that I used @inferred wrong in the tests for #2196. Because of this, the tests didn't notice that exponent_vectors(Vector{UInt}, f) did not return Vector{UInt}s.

[joschmitt]

Yay, this is breaking now. It requires signatures exponent_vector(::Type{Vector{Int}}, ::MPoly, ::Int).
Is that fine? Or should I add some (temporary?) fallback to src/MPoly.jl?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.99%. Comparing base (4c01fa1) to head (2409e84).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2221   +/-   ##
=======================================
  Coverage   87.98%   87.99%           
=======================================
  Files         127      127           
  Lines       31805    31807    +2     
=======================================
+ Hits        27985    27987    +2     
  Misses       3820     3820           

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[thofma]

I would vote for the temporary fallback, but don't have a strong opinion.