Pin the full dependency trees of the font/Fluent linter test requirements

The `requirements.txt` files of the two Python-based builds only listed
the top-level dependency, and thus not the full dependency tree, and
only limited it via a version range. This means that the actual versions
we use for the builds are determined at runtime, and can thus easily
change if a new version gets published. This causes the builds to not be
deterministic, and it's in contrast to the JavaScript-based builds where
`package-lock.json` pins the full dependency tree with fixed versions.

This commit changes the `requirements.txt` files to follow the same
approach as `package-lock.json` and thus pin the full dependency tree to
fixed versions. This ensures deterministic builds, improves consistency
and provides better protection against e.g. supply chain attacks by not
automatically pulling in new versions as they are published (but rather
make updating versions a conscious and verifiable/auditable action). To
simplify the update process, and make it repeatable, we document the
full one-line generation commands inline.

.github/fluent_linter_requirements.txt

@@ -1 +1,7 @@
-moz-fluent-linter==0.4.*
+# The requirements below can be regenerated with the following one-liner:
+# `python3 -m venv venv; source venv/bin/activate; pip install -q moz-fluent-linter; pip freeze; deactivate; rm -rf venv`
+fluent.syntax==0.19.0
+moz-fluent-linter==0.4.10
+PyYAML==6.0.3
+six==1.17.0
+typing_extensions==4.16.0

.github/font_tests_requirements.txt

@@ -1 +1,3 @@
-fonttools==4.*
+# The requirements below can be regenerated with the following one-liner:
+# `python3 -m venv venv; source venv/bin/activate; pip install -q fonttools; pip freeze; deactivate; rm -rf venv`
+fonttools==4.63.0