Since Go version 1.13, the standard library has built-in support for
wrapping and unwrapping of error values. This commit bumps the minimum
version required by the module from 1.12 to 1.13, replaces all calls to
errors.Wrap/f with fmt.Errorf and removes the now unneeded dependency on
github.com/pkg/errors.
Signed-off-by: Michael Hofmann <michael.hofmann@bitgestalt.com>
