style: format code with Black

This commit fixes the style issues introduced in d9c5297 according to the output
from Black.

Details: None

src/causalimpact/analysis.py

@@ -695,7 +695,8 @@ def _print_report(
                 """The probability of obtaining this effect by
                 chance is p = {p}. This means the effect may
                 be spurious and would generally not be considered
-                statistically significant.""".format(p=np.round(p_value, 3)
+                statistically significant.""".format(
+                    p=np.round(p_value, 3)
                 )
             )