tests: enforce the use of `from __future__ import annotations`

A recent MR and a separate recently landed MR that extracted code to a new file
overlooked this, so I think it's worth flagging to ensure consistency.  We don't
enforce the import for empty files (like `__init__.py`).

I'd rather this go into `import-checker.py`, but the import of interest only
happens at the top of the file, and its `verify_modern_convention()` calls
itself recursively as it transits the AST where the annotations might be.  After
a few hours of hacking on trying to get it to enforce the import, but only if
annotations are used in the module (we generally don't have or check annotations
in test files, so don't need this import), I gave up and resorted to this.  It
won't handle multi-line imports, but this isn't something I'd expect to change
often, so this is good enough for now.

tests/test-check-module-imports.t

@@ -1,45 +1,52
 #require test-repo hg32
 
   $ . "$TESTDIR/helpers-testrepo.sh"
   $ import_checker="$TESTDIR"/../contrib/import-checker.py
 
   $ cd "$TESTDIR"/..
 
 There are a handful of cases here that require renaming a module so it
 doesn't overlap with a stdlib module name. There are also some cycles
 here that we should still endeavor to fix, and some cycles will be
 hidden by deduplication algorithm in the cycle detector, so fixing
 these may expose other cycles.
 
 Known-bad files are excluded by -X as some of them would produce unstable
 outputs, which should be fixed later.
 
 NOTE: the `hg files` command here only works on files that are known to
 Mercurial. If you add an import of a new file and haven't yet `hg add`ed it, you
 will likely receive warnings about a direct import.
 
   $ testrepohg files 'set:**.py or grep(r"^#!.*?python")' \
   > 'glob:tests/**.t' \
   > -X hgweb.cgi \
   > -X setup.py \
   > -X contrib/automation/ \
   > -X contrib/debugshell.py \
   > -X contrib/hgweb.fcgi \
   > -X contrib/packaging/hg-docker \
   > -X contrib/packaging/hgpackaging/ \
   > -X contrib/packaging/inno/ \
   > -X contrib/phab-clean.py \
   > -X contrib/python-zstandard/ \
   > -X contrib/win32/hgwebdir_wsgi.py \
   > -X contrib/perf-utils/perf-revlog-write-plot.py \
   > -X doc/gendoc.py \
   > -X doc/hgmanpage.py \
   > -X i18n/posplit \
   > -X mercurial/thirdparty \
   > -X tests/hypothesishelpers.py \
   > -X tests/test-check-interfaces.py \
   > -X tests/test-demandimport.py \
   > -X tests/test-imports-checker.t \
   > -X tests/test-verify-repo-operations.py \
   > -X tests/test-extension.t \
   > | sed 's-\\-/-g' | "$PYTHON" "$import_checker" -
+
+All files that get type checked must have 'from __future__ import annotations'
+
+  $ testrepohg files 'set:**.py and size(">0")' -I mercurial -I hgext -X mercurial/thirdparty -0 \
+  > | xargs -0 grep -L '^from __future__ import annotations$'
+  mercurial/cmd_impls/graft.py
+  mercurial/testing/ps_util.py