Fix LaTeX output of \mathbb in math

If \mathbb is used in math, which is commonly used to represent sets,
the output LaTeX won't compile. This commit fixes this issue by adding
amsfonts package in the setup of sphinx.ext.mathbase .

sphinx/ext/mathbase.py

@@ -214,3 +214,4 @@ def setup_math(app, htmlinlinevisitors, htmldisplayvisitors):
     app.add_role('eq', eq_role)
     app.add_directive('math', MathDirective)
     app.connect('doctree-resolved', number_equations)
+    app.add_latex_package('amsfonts')

tests/root/math.txt

@@ -19,4 +19,8 @@ This is inline math: :math:`a^2 + b^2 = c^2`.
 
    e^{ix} = \cos x + i\sin x
 
+.. math::
+
+   n \in \mathbb N
+
 Referencing equation :eq:`foo`.