Prepare for Base.Lockable

[vtjnash]

c.f. JuliaLang/julia#52898

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6e06acf) 89.86% compared to head (52c748e) 89.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #31   +/-   ##
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  Coverage   89.86%   89.86%           
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  Files           8        8           
  Lines         365      365           
=======================================
  Hits          328      328           
  Misses         37       37           

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[ericphanson]

IIRC the only difference between the Lockable in this repo and the one in Base is that the one in Base does an assert_haslock when you getindex to access the value. I could see that breaking code (maybe in tests or whatever that isn't necessarily being careful). So just wanted to call out that potential gotcha here, since folks might get unexpected errors when updating to v1.11.