Release 0.15.4

Bugfix release. Principal reason for release is to update cython
version in order to fix pip install pysam with python 3.8.

[#879] Fix add_meta function in libcbcf.pyx, so meta-information
lines in header added with this function have double-quoting rules
in accordance to rules specified in VCF4.2 and VCF4.3 specifications
[#863] Force arg to bytes to support non-ASCII encoding
[#875] Bump minimum Cython version
[#868] Prevent segfault on Python 2.7 AlignedSegment.compare(other=None)
[#867] Fix wheel building on TravisCI
[#863] Force arg to bytes to support non-ASCII encoding
[#799] disambiguate interpretation of bcf_read return code
[#841] Fix silent truncation of FASTQ with bad q strings
[#846] Prevent segmentation fault on ID, when handling malformed records
[#829] Run configure with the correct CC/CFLAGS/LDFLAGS env va

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