Release 0.8.0

API-Incompatible Changes

• Replace msg argument with msg_fmt in all assertions (except fail()).
  This allows you to customize error messages more easily than before, because
  format() with appropriate keyword arguments is now called on these
  strings. See the documentation of individual assertions for the supported
  arguments.
• Replace AssertRaisesContext.msg and AssertWarnsContext.msg with msg_fmt.
• assert_almost_equal(), assert_not_almost_equal(): Place msg_fmt as third
  argument.

API Additions

• assert_count_equal(): Add msg_fmt argument.
• Add AssertRaisesErrnoContext, AssertRaisesRegexContext, and
  AssertWarnsRegexContext.