* petsc: remove dependency on boost - its required only as a dependency on trilinos
Also set COPTFLAGS='' etc - this way CFLAGS is the primary way optimization flags are set by spack
* petsc: update pkgs with petsc~boost dependency
* fix flake8
* boost is optional for trilinos - so adjust petsc dependency on boost [via trilinos] accordingly
* do not add boost dependency on petsc [due to trilinos requirement]. Trilinos adds this dependency anyway.
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