v6.0.0

What's Changed

• MyPy Migration by @AdamGagorik in
  #57
• Enable MyPy checks in CI by @AdamGagorik in
  #58
• Use mkdocs-macros to inject --help messages into docs by @AdamGagorik in
  #59
• Implement buying and selling rates by @AdamGagorik in
  #60

Full Changelog:
v5.2.1...v6.0.0