pkg: add remaining packages to requirements.txt

Also slightly reformatted cov requirements.txt.

requirements.txt

@@ -1,2 +1,7 @@
+# Hard dep
 jinja2
-pandas
+
+# Soft dep
+pandas
+matplotlib
+Pillow

requirements/coverage.txt

@@ -3,9 +3,9 @@ pytest
 pytest-cov
 
 # Package requirements
-pandas
 jinja2
 
 # Optional
+pandas
 matplotlib
 Pillow