Upgrade from 0.27.2 to 0.27.3 causes 2x slowdown

After installing `0.27.2` version, there was binary `pygit2/_libgit2.abi3.so` file created.

Timer showed:
```
$ time python main.py

real	0m0.352s
user	0m0.180s
sys     0m0.050s
```

After upgrade to `0.27.3` version, that binary file is absent and `pygit2` uses `cffi` as fallback.

Timer shows:
```
$ time python main.py

real	0m0.788s
user	0m0.470s
sys	    0m0.050s
```