CI: use mypy version from requirements for primer (#5834)

.github/workflows/mypy_primer.yml

@@ -32,6 +32,7 @@ jobs:
         shell: bash
         run: |
           cd typeshed_to_test
+          MYPY_VERSION=$(grep mypy== requirements-tests-py3.txt | cut -d = -f 3)
           echo "new commit"
           git rev-list --format=%s --max-count=1 $GITHUB_SHA
           git checkout -b upstream_master origin/master
@@ -42,7 +43,7 @@ jobs:
           # fail action if exit code isn't zero or one
           (
             mypy_primer \
-            --new v0.901 --old v0.901 \
+            --new v${MYPY_VERSION} --old v${MYPY_VERSION} \
             --custom-typeshed-repo typeshed_to_test \
             --new-typeshed $GITHUB_SHA --old-typeshed upstream_master \
             --num-shards 2 --shard-index ${{ matrix.shard-index }} \