Reformat .flake8 configuration to not error

This makes it look more like the examples in:

https://flake8.pycqa.org/en/6.0.0/user/configuration.html

The configuration was actually broken:

PyCQA/flake8#1760

Avoids:

```
ValueError: Error code '\\' supplied to 'ignore' option does not match '^[A-Z]{1,3}[0-9]{0,3}$'
```

.flake8

@@ -1,11 +1,15 @@
 [flake8]
 exclude = .direnv,.venv,venv,tests/acceptance/external_studies
-extend-select = \
-    W504  # match black&PEP8 putting binary operators after new lines
-ignore = \
-    E203 \ # whitespace before : (black disagrees)
-    E501 \ # line too long (black fixes long lines, except for long strings which may benefit from being long (eg URLs))
-    W503 \ # line break before binary operator (black disagrees)
+extend-select =
+    # match black&PEP8 putting binary operators after new line
+    W504
+ignore =
+    # whitespace before : (black disagrees)
+    E203,
+    # line too long (black fixes long lines, except for long strings which may benefit from being long (eg URLs))
+    E501,
+    # line break before binary operator (black disagrees)
+    W503,
 max-line-length = 88
 per-file-ignores =
     scripts/create-schema.py:INP001,E402