Use -r in make-changelog tox env

This is necessary because the most recent versions of pip have a bug
with using `-c` having `--require-hashes` or entries with extras.

Ref: pypa/pip#9243

tox.ini

@@ -320,8 +320,12 @@ commands =
     --version \
     {posargs:'[UNRELEASED DRAFT]' --draft}
 deps =
-  towncrier
-  -c{toxinidir}/docs/requirements.txt
+  -r{toxinidir}/docs/requirements.txt
+  # FIXME: re-enable the "-r" + "-c" paradigm once the pip bug is fixed.
+  # Ref: https://github.com/pypa/pip/issues/9243
+  # towncrier
+  # -r{toxinidir}/docs/requirements.in
+  # -c{toxinidir}/docs/requirements.txt
 isolated_build = true
 skip_install = true