Use some handy bash-isms in version check script

The script is nonportable to other shells already because of its
use of trap (and other features, such as implicit assumptions made
about echo, and the function keyword), which its hashbang already
clearly conveys.

This uses:

- $(<X) in place of $(cat X), to have the shell read the file
  directly rather than using cat.

- printf -v in one place to set a variable rather than printing.
  This eliminates a command substitution that was harder to read.

check-version.sh

@@ -12,7 +12,7 @@ readonly changes_path='doc/source/changes.rst'
 
 function get_latest_tag() {
     local config_opts
-    config_opts="$(printf ' -c versionsort.suffix=-%s' alpha beta pre rc RC)"
+    printf -v config_opts ' -c versionsort.suffix=-%s' alpha beta pre rc RC
     # shellcheck disable=SC2086  # Deliberately word-splitting the arguments.
     git $config_opts tag -l '[0-9]*' --sort=-v:refname | head -n1
 }
@@ -31,7 +31,7 @@ echo 'Checking that ALL changes are committed.'
 git status -s --ignore-submodules
 test -z "$(git status -s --ignore-submodules)"
 
-version_version="$(cat "$version_path")"
+version_version="$(<"$version_path")"
 changes_version="$(awk '/^[0-9]/ {print $0; exit}' "$changes_path")"
 latest_tag="$(get_latest_tag)"
 head_sha="$(git rev-parse HEAD)"