Pin pdm version in ci/cd due to pypy regression in 2.26.3

A new 2.26.3 pdm version does not play well with pypy:
pdm-project/pdm#3706

Author: olzhasar-reef

.github/workflows/cd.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Display Python version
         run: python -c "import sys; print(sys.version)"
       - name: Install dependencies
-        run: python -m pip install --upgrade nox pdm
+        run: python -m pip install --upgrade nox pdm==2.26.2
       - name: Build the distribution
         id: build
         run: nox -vs build

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
           python-version: ${{ env.PYTHON_DEFAULT_VERSION }}
           cache: "pip"
       - name: Install dependencies
-        run: python -m pip install --upgrade nox pdm
+        run: python -m pip install --upgrade nox pdm==2.26.2
       - name: Run linters
         run: nox -vs lint
       - name: Validate new changelog entries
@@ -49,7 +49,7 @@ jobs:
           python-version: ${{ env.PYTHON_DEFAULT_VERSION }}
           cache: "pip"
       - name: Install dependencies
-        run: python -m pip install --upgrade nox pdm
+        run: python -m pip install --upgrade nox pdm==2.26.2
       - name: Build the distribution
         run: nox -vs build
   cleanup_buckets:
@@ -72,7 +72,7 @@ jobs:
           cache: "pip"
       - name: Install dependencies
         if: ${{ env.B2_TEST_APPLICATION_KEY != '' && env.B2_TEST_APPLICATION_KEY_ID != '' }}  # TODO: skip this whole job instead
-        run: python -m pip install --upgrade nox pdm
+        run: python -m pip install --upgrade nox pdm==2.26.2
       - name: Find and remove old buckets
         if: ${{ env.B2_TEST_APPLICATION_KEY != '' && env.B2_TEST_APPLICATION_KEY_ID != '' }}  # TODO: skip this whole job instead
         run: nox -vs cleanup_old_buckets
@@ -109,7 +109,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
           cache: "pip"
       - name: Install dependencies
-        run: python -m pip install --upgrade nox pdm
+        run: python -m pip install --upgrade nox pdm==2.26.2
       - name: Run unit tests
         run: nox -vs unit -- -v
       - name: Run integration tests
@@ -134,6 +134,6 @@ jobs:
         run: |
           sudo apt-get update -y
           sudo apt-get install -y graphviz plantuml
-          python -m pip install --upgrade nox pdm
+          python -m pip install --upgrade nox pdm==2.26.2
       - name: Build the docs
         run: nox --non-interactive -vs doc