Doctest failure with Julia nightly due to use of MSet in `character_degrees` tests

[fingolfin]

This is not super urgent right now but we should take care of it eventually.

page: src/Groups/group_characters.jl:1870-1877
┌ Error: doctest failure in src/Groups/group_characters.jl:1870-1877
│ 
│ ```jldoctest
│ julia> character_degrees(character_table("S5"))
│ MSet{ZZRingElem} with 7 elements:
│   5 : 2
│   4 : 2
│   6
│   1 : 2
│ ```
│ 
│ Subexpression:
│ 
│ character_degrees(character_table("S5"))
│ 
│ Evaluated output:
│ 
│ MSet{ZZRingElem} with 7 elements:
│   1 : 2
│   4 : 2
│   5 : 2
│   6
│ 
│ Expected output:
│ 
│ MSet{ZZRingElem} with 7 elements:
│   5 : 2
│   4 : 2
│   6
│   1 : 2
│ 
│   diff =
│    MSet{ZZRingElem} with 7 elements:
│      5 1 : 2
  0.406529 seconds (746.28 k allocations: 37.478 MiB, 99.07% compilation time: 1% of which was recompilation)
│      4 : 2
│      6
│      1 5 : 22
│      6
└ @ Documenter ~/work/Oscar.jl/Oscar.jl/src/Groups/group_characters.jl:1870
┌ Error: doctest failure in src/Groups/group_characters.jl:1906-1921
│ 
│ ```jldoctest
│ julia> character_degrees([3, 3, 5], 2)
│ MSet{ZZRingElem} with 14 elements:
│   4 : 9
│   2 : 4
│   1
│ 
│ julia> character_degrees([3, 3, 5], 4)
│ MSet{ZZRingElem} with 27 elements:
│   2 : 18
│   1 : 9
│ 
│ julia> character_degrees([3, 3, 5], 16)
│ MSet{ZZRingElem} with 45 elements:
│   1 : 45
│ ```
│ 
│ Subexpression:
│ 
│ character_degrees([3, 3, 5], 2)
│ 
│ Evaluated output:
│ 
│ MSet{ZZRingElem} with 14 elements:
│   2 : 4
│   1
│   4 : 9
│ 
│ Expected output:
│ 
│ MSet{ZZRingElem} with 14 elements:
│   4 : 9
│   2 : 4
│   1
│ 
│   diff =
│    MSet{ZZRingElem} with 14 elements:
│      4 : 9
│      2 : 4
│      11
│      4 : 9
└ @ Documenter ~/work/Oscar.jl/Oscar.jl/src/Groups/group_characters.jl:1906