Lock pyasn1<0.5.0

`snmpsim` depends on `pyasn1`, but does not lock the dependent
version. `pyasn1` version 0.5.0 was recently released, and it seems
`snmpsim` is incompatible with the new version, causing integration
tests that use snmpsim to fail.

This locks pyasn1 to a version that should still work with the latest
version of snmpsim.

.github/workflows/build-and-test.yml

@@ -49,7 +49,7 @@ jobs:
       - name: "Install test runner dependencies"
         run: |
           set -xe
-          python -m pip install --upgrade pip setuptools wheel 'tox<4' tox-gh-actions coverage virtualenv snmpsim
+          python -m pip install --upgrade pip setuptools wheel 'tox<4' tox-gh-actions coverage virtualenv snmpsim 'pyasn1<0.5.0'
           sudo apt-get install -y nbtscan
 
       # virtualenv seems to currently be embedding a broken version of

tests/docker/Dockerfile

@@ -65,7 +65,7 @@ RUN cd /tmp && \
     mv chromedriver /usr/local/bin/
 
 # Install our primary test runner
-RUN python3.7 -m pip install 'tox<4' snmpsim virtualenv
+RUN python3.7 -m pip install 'tox<4' snmpsim 'pyasn1<0.5.0' virtualenv
 
 # Add a build user
 ENV USER build