Fix flake8 config and adhere code to flake8

Author: Christian Knittl-Frank

.flake8

@@ -1,7 +1,7 @@
 [flake8]
 max-line-length = 88
 extend-ignore =
-    E203  # "Whitespace before ':'" - not PEP-8 compliant
-    E501  # "Line too long (82 >= 79 characters)"
+    E203
+    E501
 per-file-ignores =
     __init__.py:F401

src/sphinxcontrib/email/handlers.py

@@ -33,8 +33,8 @@ def html_page_context_handler(
 
         old_node.getparent().replace(old_node, new_node)
 
-    child_strs = map(
-        lambda child: lxml.html.tostring(child, pretty_print=True, encoding="unicode"),
-        tree.iterchildren(),
-    )
+    child_strs = [
+        lxml.html.tostring(child, pretty_print=True, encoding="unicode")
+        for child in tree.iterchildren()
+    ]
     context["body"] = "".join(child_strs)