src/sage/modular/modform/element.py: add "needs sage.libs.eclib"

One example in this file requires "needs sage.libs.eclib" to pass
when eclib is not installed (or is disabled).

Author: orlitzky

src/sage/modular/modform/element.py

@@ -926,6 +926,7 @@ def period(self, M, prec=53):
         These can be used to express the periods of `f` as exact
         linear combinations of the real and the imaginary period of `E`::
 
+            sage: # needs sage.libs.eclib
             sage: s = E.modular_symbol(sign=+1)
             sage: t = E.modular_symbol(sign=-1, implementation='sage')
             sage: s(3/11), t(3/11)