obsolete: add readonly flag to obstore constructor Previously, obstore read the obsolete._enabled flag to determine whether to allow writes to the obstore. Since obsolete._enabled will be moving into a repo specific config, we can't read it globally, and therefore must pass the information into the constructor.

obsolete: add isenabled function for option checking Previously, obsolete used the module level _enabled flag to determine whether it was on or off. We need a bit more granular control, so we'll be introducing toggle options. The isenabled() function is how you check if a particular option is enabled for the given repository. Future patches will add options such as 'createmarkers', 'allowunstable', and 'exchange' to enable various features of obsolete markers.

obsstore: fix defaultformat option passing The obsstore format passing was not actually being passed to the obsstore. This fixes it.

tests: use $PYTHON instead of hardcoding python This makes running the testsuite with pypy possible.