ci: pin flake8 version

- new version of flake8-bugbear (24.1.16) introduced new tests (B036), which we were failing
- that's fine, but our CI should not suddenly break because of this
- better to do explicit manual version bumps of the linter

.cirrus.yml

@@ -128,7 +128,7 @@ task:
     folder: ~/.cache/pip
     fingerprint_script: echo Flake8 && echo $ELECTRUM_IMAGE && cat $ELECTRUM_REQUIREMENTS
   install_script:
-    - pip install flake8 flake8-bugbear
+    - pip install "flake8==7.0.0" "flake8-bugbear==24.1.16"
   flake8_script:
     - flake8 . --count --select="$ELECTRUM_LINTERS" --ignore="$ELECTRUM_LINTERS_IGNORE" --show-source --statistics --exclude "*_pb2.py,electrum/_vendor/"
   env:
@@ -142,7 +142,7 @@ task:
         # - https://pycodestyle.pycqa.org/en/latest/intro.html#error-codes
         # - https://github.com/PyCQA/flake8-bugbear/tree/8c0e7eb04217494d48d0ab093bf5b31db0921989#list-of-warnings
         ELECTRUM_LINTERS: E9,E101,E129,E273,E274,E703,E71,E722,F63,F7,F82,W191,W29,B
-        ELECTRUM_LINTERS_IGNORE: B007,B009,B010,B019
+        ELECTRUM_LINTERS_IGNORE: B007,B009,B010,B019,B036
     - name: "linter: Flake8 Non-Mandatory"
       env:
         ELECTRUM_LINTERS: E,F,W,C90,B