Updates for JuMP 0.21

[odow]

Blockers

• Update Project.toml

[codecov]

Codecov Report

Merging #300 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #300      +/-   ##
=========================================
+ Coverage   94.14%   94.2%   +0.05%     
=========================================
  Files          20      20              
  Lines        1691    1690       -1     
=========================================
  Hits         1592    1592              
+ Misses         99      98       -1

Impacted Files	Coverage Δ
src/user_interface.jl	98.38% <ø> (ø)	⬆️
src/plugins/integrality_handlers.jl	97.26% <ø> (ø)	⬆️
src/deterministic_equivalent.jl	99% <ø> (ø)	⬆️
src/plugins/headers.jl	100% <100%> (+16.66%)	⬆️

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