Revert to treating exclude in .ini as single string (python#11881)

Partially reverts python#11329 (with respect to `.ini` documentation).
Modifies existing unit tests.

Fixes python#11830.

Co-authored-by: Matthew W <matthew@willcockson.family>

docs/source/config_file.rst

@@ -197,18 +197,28 @@ section of the command line docs.
 
 .. confval:: exclude
 
-    :type: newline separated list of regular expressions
+    :type: regular expression
 
-    A newline list of regular expression that matches file names, directory names and paths
+    A regular expression that matches file names, directory names and paths
     which mypy should ignore while recursively discovering files to check.
     Use forward slashes on all platforms.
 
     .. code-block:: ini
 
       [mypy]
-      exclude =
-          ^file1\.py$
-          ^file2\.py$
+      exclude = (?x)(
+          ^one\.py$    # files named "one.py"
+          | two\.pyi$  # or files ending with "two.pyi"
+          | ^three\.   # or files starting with "three."
+        )
+
+    Crafting a single regular expression that excludes multiple files while remaining
+    human-readable can be a challenge. The above example demonstrates one approach.
+    ``(?x)`` enables the ``VERBOSE`` flag for the subsequent regular expression, which
+    `ignores most whitespace and supports comments`__. The above is equivalent to:
+    ``(^one\.py$|two\.pyi$|^three\.)``.
+
+    .. __: https://docs.python.org/3/library/re.html#re.X
 
     For more details, see :option:`--exclude <mypy --exclude>`.
 

mypy/config_parser.py

@@ -132,7 +132,7 @@ def check_follow_imports(choice: str) -> str:
     'cache_dir': expand_path,
     'python_executable': expand_path,
     'strict': bool,
-    'exclude': lambda s: [p.strip() for p in s.split('\n') if p.strip()],
+    'exclude': lambda s: [s.strip()],
 }
 
 # Reuse the ini_config_types and overwrite the diff

test-data/unit/cmdline.test

@@ -1354,9 +1354,10 @@ b/bpkg.py:1: error: "int" not callable
 # cmd: mypy .
 [file mypy.ini]
 \[mypy]
-exclude =
-    abc
-    b
+exclude = (?x)(
+    ^abc/
+    |^b/
+  )
 [file abc/apkg.py]
 1()
 [file b/bpkg.py]