pysam-0.9.1

This is a bugfix release addressing some installation problems
in pysam 0.9.0, in particular:

• patch included htslib to work with older libcurl versions, fixes #262.
• do not require cython for python 3 install, fixes #260
• FastaFile does not accept filepath_index any more, see #270
• add AlignedSegment.get_cigar_stats method.
• py3 bugfix in VariantFile.subset_samples, fixes #272
• add missing sysconfig import, fixes #278
• do not redirect stdout, but instead write to a separately
  created file. This should resolve issues when pysam is used
  in notebooks or other environments that redirect stdout.
• wrap htslib-1.3.1, samtools-1.3.1 and bcftools-1.3.1
• use bgzf throughout instead of gzip
• allow specifying a fasta reference for CRAM file when opening
  for both read and write, fixes #280