Require `[compat]` entry for `julia`

[lgoettgens]

Resolves https://github.com/JuliaTesting/Aqua.jl/issues/235.

As this is point 7 in the guidelines for the General registry, I support Aqua checking this.

[codecov[bot]]

Codecov Report

Merging #236 (cdede79) into master (3ea2897) will decrease coverage by 2.08%. Report is 1 commits behind head on master. The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
- Coverage   76.17%   74.09%   -2.08%     
==========================================
  Files          11       11              
  Lines         768      776       +8     
==========================================
- Hits          585      575      -10     
- Misses        183      201      +18     

Flag	Coverage Δ
unittests	74.09% <89.47%> (-2.08%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/project_extras.jl	98.00% <100.00%> (ø)
src/deps_compat.jl	73.07% <88.88%> (+2.80%)	:arrow_up:

... and 6 files with indirect coverage changes

:mega: Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[lgoettgens]

@fingolfin Do you wanna have a look at this? If not, I will just merge this and https://github.com/JuliaTesting/Aqua.jl/pull/223, and then release 0.8.0.