test: Fix recursion issue in walkpdf (#1112)

This fixes the recursion issue in walkpdf introduced by PyPDF==4.1.0.

This fix is based @pubpub-zz's suggestion in
py-pdf/pypdf#2508 (comment)

Fixes #1111

.github/workflows/checks.yml

@@ -90,7 +90,7 @@ jobs:
         echo "Installing pip + wheel..."
         python -m pip install --upgrade pip wheel
         echo "Installing requirements.txt + test dependencies..."
-        python -m pip install -r requirements.txt tox tox-gh-actions certifi decorator dict2xml pyflakes "pypdf>=3.2.1" "weasyprint>=53.0,!=57.0,!=60.0"
+        python -m pip install -r requirements.txt tox tox-gh-actions certifi decorator dict2xml pyflakes "pypdf>=4.1.0" "weasyprint>=53.0,!=57.0,!=60.0"
         
     - name: Generate Valid Tests
       run: |
@@ -151,7 +151,7 @@ jobs:
         echo "Installing pip + wheel..."
         python -m pip install --upgrade pip wheel
         echo "Installing requirements.txt + test dependencies..."
-        python -m pip install -r requirements.txt tox tox-gh-actions certifi decorator dict2xml pyflakes "pypdf>=3.2.1" "weasyprint>=53.0,!=57.0,!=60.0"
+        python -m pip install -r requirements.txt tox tox-gh-actions certifi decorator dict2xml pyflakes "pypdf>=4.1.0" "weasyprint>=53.0,!=57.0,!=60.0"
         
     - name: Generate Valid Tests
       run: |
@@ -201,7 +201,7 @@ jobs:
 #         echo "Installing pip + wheel..."
 #         python -m pip install --upgrade pip wheel
 #         echo "Installing requirements.txt + test dependencies..."
-#         python -m pip install -r requirements.txt tox tox-gh-actions certifi decorator dict2xml pyflakes "pypdf>=3.2.1" "weasyprint>=53.0,!=57.0,!=60.0"
+#         python -m pip install -r requirements.txt tox tox-gh-actions certifi decorator dict2xml pyflakes "pypdf>=4.1.0" "weasyprint>=53.0,!=57.0,!=60.0"
 #     - name: Generate Valid Tests
 #       run: |
 #         make yestests || true

tox.ini

@@ -33,5 +33,5 @@ deps =
     pyflakes
     decorator
     dict2xml
-    pypdf>=3.2.1
+    pypdf>=4.1.0
     weasyprint>=53.0,!=57.0,!=60.0

xml2rfc/walkpdf.py

@@ -15,7 +15,7 @@
 def walk(obj, seen):
     dobj = {}                            # Direct objects
     iobj = []                            # Indirect objects
-    if hasattr(obj, 'keys'):
+    if isinstance(obj, pypdf.generic.DictionaryObject):
         for key in obj.keys():
             k = key[1:] if key.startswith('/') else key
             d, i = walk(obj[key], seen)