[FIX] Relax sequence_file_input_traits

[eseiler]

explicitly_convertible_to<typename t::sequence_legal_alphabet, typename t::sequence_alphabet> is meant for allowing, e.g.:

using sequence_alphabet = seqan3::dna4;
using sequence_legal_alphabet = seqan3::dna15; // or even seqan3::rna15

but not stuff like

using sequence_alphabet = seqan3::dna4;
using sequence_legal_alphabet = seqan3::aa27;

However, this constraint is also checked for using sequence_alphabet = char. Our alphabets are not (explicitly/implicitly) convertible to char.
The rationale of the alphabets being compatible doesn't apply for sequence_alphabet = char, so we should just allow it.

using sequence_alphabet = char;
using sequence_legal_alphabet = seqan3::dna4;

means: Only allow characters of seqan3::dna4, but store them as char.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
seqan3	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 25, 2023 10:30am

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (aa502ef) 98.18% compared to head (11dc477) 98.18%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3128   +/-   ##
=======================================
  Coverage   98.18%   98.18%           
=======================================
  Files         276      276           
  Lines       12294    12294           
=======================================
  Hits        12071    12071           
  Misses        223      223           

Impacted Files	Coverage Δ
include/seqan3/io/sequence_file/input.hpp	100.00% <ø> (ø)

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