Enable conversion of CompoundPeriod to Period

[omus]

Ref: JuliaLang/julia#40803

[codecov]

Codecov Report

Merging #754 (241c018) into master (cf3426b) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #754      +/-   ##
==========================================
+ Coverage   78.13%   78.20%   +0.07%     
==========================================
  Files           4        4              
  Lines         622      624       +2     
==========================================
+ Hits          486      488       +2     
  Misses        136      136              

Impacted Files	Coverage Δ
src/Compat.jl	78.12% <100.00%> (+0.08%)	⬆️

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