add missing optional tags, fix linter

Author: fchapoton

src/sage/rings/laurent_series_ring.py

@@ -425,6 +425,7 @@ def _magma_init_(self, magma):
             sage: S.sage() == R
             True
 
+            sage: # optional - magma
             sage: magma(LaurentSeriesRing(GF(7), 'x'))                                     # needs sage.rings.finite_rings
             Laurent series field in x over GF(7)
         """

src/sage/rings/power_series_ring.py

@@ -739,7 +739,8 @@ def _magma_init_(self, magma):
             y
             sage: S.sage() == R
             True
-        
+
+            sage: # optional - magma
             sage: magma(PowerSeriesRing(GF(7), 'x'))                                     # needs sage.rings.finite_rings
             Power series ring in x over GF(7)
         """

src/sage/rings/puiseux_series_ring.py

@@ -295,6 +295,7 @@ def _magma_init_(self, magma):
             sage: S.sage() == R
             True
 
+            sage: # optional - magma
             sage: magma(PuiseuxSeriesRing(GF(7), 'x'))                                     # needs sage.rings.finite_rings
             Puiseux series field in x over GF(7)
         """