Fix PEP-8 issues in contributing.rst (#24003)

Author: FHaase

doc/source/contributing.rst

@@ -670,6 +670,8 @@ Otherwise, you need to do it manually:
 
 .. code-block:: python
 
+    import warnings
+
     def old_func():
         """Summary of the function.
 
@@ -679,6 +681,9 @@ Otherwise, you need to do it manually:
         warnings.warn('Use new_func instead.', FutureWarning, stacklevel=2)
         new_func()
 
+    def new_func():
+        pass
+
 You'll also need to
 
 1. write a new test that asserts a warning is issued when calling with the deprecated argument
@@ -933,6 +938,8 @@ If your change involves checking that a warning is actually emitted, use
 
 .. code-block:: python
 
+   df = pd.DataFrame()
+
    with tm.assert_produces_warning(FutureWarning):
        df.some_operation()
 
@@ -963,7 +970,7 @@ a single test.
 
 .. code-block:: python
 
-   with warch.catch_warnings():
+   with warnings.catch_warnings():
        warnings.simplefilter("ignore", FutureWarning)
        # Or use warnings.filterwarnings(...)
 

setup.cfg

@@ -73,7 +73,6 @@ exclude =
     doc/source/comparison_with_sql.rst
     doc/source/comparison_with_stata.rst
     doc/source/computation.rst
-    doc/source/contributing.rst
     doc/source/contributing_docstring.rst
     doc/source/dsintro.rst
     doc/source/enhancingperf.rst