Python 0.5.7

Breaking Changes

• The VCF writing methods (ts.write_vcf, ts.as_vcf) now error if a site with
  position zero is encountered. The VCF spec does not allow zero position sites.
  Suppress this error with the allow_position_zero argument.
  (@benjeffery, #2901, #2838)

Bugfixes

• Fix to the folded, expected allele frequency spectrum (i.e.,
  TreeSequence.allele_frequency_spectrum(mode="branch", polarised=False),
  which was half as big as it should have been. (@petrelharp,
  @nspope, #2933)