Issue #27626: Spelling fixes in docs, comments and internal names

Based on patch by Ville Skyttä.
diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 0c23c96..8f46101 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -529,7 +529,7 @@
            between the two objects is more than the given delta.
 
            Note that decimal places (from zero) are usually not the same
-           as significant digits (measured from the most signficant digit).
+           as significant digits (measured from the most significant digit).
 
            If the two objects compare equal then they will automatically
            compare almost equal.
@@ -567,7 +567,7 @@
            between the two objects is less than the given delta.
 
            Note that decimal places (from zero) are usually not the same
-           as significant digits (measured from the most signficant digit).
+           as significant digits (measured from the most significant digit).
 
            Objects that are equal automatically fail.
         """