test_runner: fix mypy errors and force it on CI (#774)

* Fix bugs found by mypy
* Add some missing types and runtime checks, remove unused code
* Make ZenithPageserver start right away for better type safety
* Add `types-*` packages to Pipfile
* Pin mypy version and run it on CircleCI

test_runner/Pipfile

@@ -14,11 +14,14 @@ asyncpg = "*"
 cached-property = "*"
 
 [dev-packages]
-flake8 = "*"
-mypy = "*"
 # Behavior may change slightly between versions. These are run continuously,
 # so we pin exact versions to avoid suprising breaks. Update if comfortable.
 yapf = "==0.31.0"
+mypy = "==0.910"
+# Non-pinned packages follow.
+flake8 = "*"
+types-requests = "*"
+types-psycopg2 = "*"
 
 [requires]
 # we need at least 3.6, but pipenv doesn't allow to say this directly