Fix syntax errors in docstring code snippets:

Some of the code examples in the docstrings where not valid Python code, so I
wrote a small test to ensure all the checks have valid Python code. This
doesn't do anything fancy like checking for imports (though maybe it should),
it merely checks that the AST tree can be built.

docs/checks.md

@@ -601,7 +601,7 @@ if x == y and x == z:
 
 # and
 
-if x is None and y is None
+if x is None and y is None:
     pass
 ```
 
@@ -2150,7 +2150,7 @@ Bad:
 from hashlib import sha512
 
 h = sha512()
-h.update(b"data)
+h.update(b"data")
 ```
 
 Good:
@@ -2172,7 +2172,7 @@ Bad:
 
 ```python
 nums = [123, 456]
-num = f"{num[0]}")
+num = f"{num[0]}"
 ```
 
 Good:

refurb/checks/hashlib/simplify_ctor.py

@@ -30,7 +30,7 @@ class ErrorInfo(Error):
     from hashlib import sha512
 
     h = sha512()
-    h.update(b"data)
+    h.update(b"data")
     ```
 
     Good:

refurb/checks/logical/use_equal_chain.py

@@ -20,7 +20,7 @@ class ErrorInfo(Error):
 
     # and
 
-    if x is None and y is None
+    if x is None and y is None:
         pass
     ```
 

refurb/checks/readability/use_str_func.py

@@ -18,7 +18,7 @@ class ErrorInfo(Error):
 
     ```
     nums = [123, 456]
-    num = f"{num[0]}")
+    num = f"{num[0]}"
     ```
 
     Good:

test/test_check_formatting.py

@@ -1,6 +1,10 @@
+import ast
 import re
 from functools import cache
 from pathlib import Path
+from textwrap import dedent
+
+import pytest
 
 import refurb
 from refurb.error import Error
@@ -55,6 +59,26 @@ def get_categories_from_docs() -> list[str]:
         return categories
 
 
+def assert_python_snippets_are_parseable(error: type[Error]) -> None:
+    # This check is simple enough that it doesn't need detailed docs
+    if error.code == 105:
+        return
+
+    docs = dedent(error(0, 0, "").__doc__ or "")
+
+    groups = re.findall(r"```([\S\s]*?)```", docs)
+    assert groups, "Missing docstring"
+
+    assert len(groups) >= 2, 'Expected at least 2 "good" and "bad" code examples in docstring'
+
+    try:
+        for code in groups:
+            ast.parse(code.strip())
+
+    except SyntaxError as ex:
+        pytest.fail(f"Python code in docs is invalid: {ex.msg}")
+
+
 def test_checks_are_formatted_properly() -> None:
     error_names: set[str] = set()
 
@@ -68,6 +92,7 @@ def test_checks_are_formatted_properly() -> None:
             assert_category_exists(error)
             assert_categories_are_sorted(error)
             assert_categories_are_valid(error, get_categories_from_docs())
+            assert_python_snippets_are_parseable(error)
 
             assert error.name, "name field missing for class"