Improve line-ending consistency in requirements files

`requirements-dev.txt`, but none of the others, was tracked with
Windows-style (CRLF) line endings. This appears to have been the
case since it was introduced in a1b7634 (as `dev-requirements.txt`)
and not to be intentional.

This only changes how it is stored in the repository. This does not
change `.gitattributes` (it is not forced to have LF line endings
if automatic line-ending conversions are configured in Git).

requirements-dev.txt

@@ -1,8 +1,8 @@
--r requirements.txt
--r test-requirements.txt
-
-# For additional local testing/linting - to be added elsewhere eventually.
-ruff
-shellcheck
-pytest-icdiff
-# pytest-profiling
+-r requirements.txt
+-r test-requirements.txt
+
+# For additional local testing/linting - to be added elsewhere eventually.
+ruff
+shellcheck
+pytest-icdiff
+# pytest-profiling