alembic: mypy failure with --no-implicit-reexport

Describe the bug

Using mypy with --no-implicit-reexport (which is part of --strict) generate a failure since alembic 1.7

Expected behavior

No AssertionError

To Reproduce

$ cat << EOF > t.py
import alembic
EOF
$ mypy --no-implicit-reexport t.py
Traceback (most recent call last):
  File "[...]/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "[...]/lib/python3.9/site-packages/mypy/__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy/main.py", line 87, in main
  File "mypy/main.py", line 165, in run_build
  File "mypy/build.py", line 179, in build
  File "mypy/build.py", line 254, in _build
  File "mypy/build.py", line 2697, in dispatch
  File "mypy/build.py", line 3021, in process_graph
  File "mypy/build.py", line 3138, in process_stale_scc
  File "mypy/build.py", line 2288, in write_cache
  File "mypy/build.py", line 1475, in write_cache
  File "mypy/nodes.py", line 313, in serialize
  File "mypy/nodes.py", line 3149, in serialize
  File "mypy/nodes.py", line 3083, in serialize
AssertionError: Definition of alembic.runtime.migration.EnvironmentContext is unexpectedly incomplete

Versions.

• OS: Debian 11
• Python: 3.9
• Alembic: 1.7.1
• SQLAlchemy: 1.4.23
• mypy: 0.910

Note it might be a mypy bug (since this is an AssertionError), but I think this looks related to some weirdness in alembic typing code.

Thanks!