Cyvcf2

+ raise exception (instead of segfault) when calling num_called, etc. on variant without genotypes (262 from davmlaw)

+ fix num_unknown when gts012=True (236)

+ use warnings instead of sys.stderr (229 from grahamgower)
+ use libdeflate in wheel build (231 from grahamgower)
+ use pytest instead of nose and update numpy stuff (232 from grahamgower)

+ fixes for mixed ploidy samples affecting `variant.num_het`,
`variant.num_hom_ref`, etc. Also affects `variant.genotypes` and
`variant.genotype` (see 227. Thanks davmlaw and grahamgower for
test-cases and insight.)

+ add variant.FILTERS (by tomwhite, 149)

+ bump for CI