try building with old rust

[samuelcolvin]

Trying building with ancient rust to hopfully fix piwheels builds, ref #149.

[codecov]

Codecov Report

Merging #150 (9c4fc91) into main (2b1ff0a) will decrease coverage by 0.54%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##              main     #150      +/-   ##
===========================================
- Coverage   100.00%   99.45%   -0.55%     
===========================================
  Files            6        6              
  Lines          368      368              
  Branches        76       76              
===========================================
- Hits           368      366       -2     
- Misses           0        1       +1     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
watchfiles/main.py	97.82% <0.00%> (-2.18%)	⬇️

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