Auto merge of #16338 - Wafflespeanut:byte, r=jdm

Disable bytecode generation in mach

Recently, [@]froydnj had some trouble with `mach`, and after half an hour of struggle, a simple `git clean` and `git reset --hard` seemed to fix the issue. Apparently, sometimes (though I'm not sure when, and I don't even have a convincing argument), the changes to `mach` scripts don't really seem to reflect on the related bytecode files (when it absolutely should've!).

Having bytecode files is just a matter of optimization (i.e., we don't have to compile the scripts again), but when it comes to our simple build system, "not having them" doesn't add a great overhead. So, we could just disable their creation to avoid such issues in the future.

Existing users can do `find ./python -name '*.pyc' -delete` for a cleanup (and maybe remove the `python/_virtualenv` directory too, though not necessary)

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mach

@@ -25,6 +25,7 @@ def main(args):
 
 
 if __name__ == '__main__':
+    sys.dont_write_bytecode = True
     if sys.platform == 'win32':
         # This is a complete hack to work around the fact that Windows
         # multiprocessing needs to import the original module (ie: this