Log Message:
tex-mathtools: update to 1.21

1.21
* Fixed typo in \intertext, the interface for specifying the space above
  and below now works as advertised.

1.20
* fixed \null\, bug in (l|r|)gathered versions, bringing it in line with
amsmath
  Reported in https://tex.stackexchange.com/a/397718/3929