refactor(build): update fortran standard to 2018

[mjreno]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.124%. Comparing base (516b61e) to head (846fcf8).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##           develop      #195       +/-   ##
=============================================
- Coverage   84.181%   84.124%   -0.058%     
=============================================
  Files           20        20               
  Lines         3496      3496               
=============================================
- Hits          2943      2941        -2     
- Misses         553       555        +2     

Files	Coverage Δ
pymake/utils/_compiler_switches.py	79.361% <ø> (ø)
pymake/utils/_meson_build.py	90.076% <100.000%> (ø)

... and 1 file with indirect coverage changes

[mjreno]

Will reopen.