Improve PEP8 adherance using flake8 linter

src/pg_rad/isotopes/isotope.py

@@ -5,7 +5,7 @@ class Isotope:
         name (str): Full name (e.g. Caesium-137).
         E (float): Energy of the primary gamma in keV.
         b (float): Branching ratio for the gamma at energy E.
-    """  
+    """
     def __init__(
             self,
             name: str,
@@ -20,4 +20,4 @@ class Isotope:
 
         self.name = name
         self.E = E
-        self.b = b
+        self.b = b