Implement a cache for cpu_info

[FrancescAlted]

Fixes #349

This provides a fix for #349

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[FrancescAlted]

This seems to work, and import time has been reduced from 1.6s to 0.3s on my box:

faltet@Mac-mini-de-Francesc ~/b/python-blosc2 (FrancescAlted/cpu-info-cache)> rm $HOME/.blosc2-cpuinfo.json                                        > time python -c"import blosc2"

________________________________________________________
Executed in    1.59 secs      fish           external
   usr time  810.65 millis    0.23 millis  810.42 millis
   sys time  325.02 millis    1.40 millis  323.62 millis

> time python -c"import blosc2"

________________________________________________________
Executed in  306.25 millis    fish           external
   usr time  235.43 millis    0.20 millis  235.23 millis
   sys time   67.69 millis    1.35 millis   66.34 millis