contrib: propagate `pytype` failures outside of `check-pytype.sh`

A recent series got landed with a pytype failure, because it wasn't propagated
to the CI caller (see c47fe7fd312d).  I suspect it started recently with
069735062524, because failures have been flagged in the past.

The shebang line needs to be specific to bash, otherwise it ignores this
non-POSIX extension.  I'm not aware of a POSIX flavor of this option.

contrib/check-pytype.sh

@@ -1,7 +1,8
-#!/bin/sh
+#!/bin/bash
 
 set -e
 set -u
+set -o pipefail
 
 cd "$(hg root)"
 

contrib/heptapod-ci.yml

@@ -442,7 +442,7 check-pytype:
       - ./contrib/setup-pytype.sh
     script:
       - echo "Entering script section"
-      - sh contrib/check-pytype.sh
+      - bash contrib/check-pytype.sh
 
 # `sh.exe --login` sets a couple of extra environment variables that are defined
 # in the MinGW shell, but switches CWD to /home/$username.  The previous value