fix: Avoid a Sphinx warning when a Latex package is already added (#10)

* Avoid a Sphinx warning when a Latex package is already added

I use this extension and another extension which adds `ulem`. My Sphinx build gives: `WARNING: latex package 'ulem' already included`.

* Add changelog entry

* Update docs/changelog.rst

---------

Co-authored-by: Shengyu Zhang <reg@silverrainz.me>

Author: adamtheturtle

docs/changelog.rst

@@ -19,6 +19,13 @@ Change Log
 
    Change log here.
 
+1.4
+---
+
+.. version:: _
+
+   - Avoid an error which occurred when another extension has already added LaTeX packages.
+
 1.3
 ---
 

src/sphinxnotes/strike/__init__.py

@@ -78,7 +78,11 @@ def setup(app: Sphinx):
     """Sphinx extension entrypoint."""
     meta.pre_setup(app)
 
-    app.add_latex_package('ulem', 'normalem')
+    latex_packages = ('ulem', 'normalem')
+    for latex_package in latex_packages:
+        # If the package is already added and we add it again, Sphinx will warn.
+        if not app.has_latex_package(latex_package):
+            app.add_latex_package(latex_package)
 
     app.add_node(
         strike_node,