Teach Mypy about sqlalchemy's declarative_base()

[thopre]

Summary

Mypy complains about alias for
sqlalchemy.ext.declarative.declarative_base() and everywhere where it is
used. SQLAlchemy has a stubs to tell Mypy about those which gets rid of
lots of Mypy warnings.

[PrzemekWirkus]

LGTM

[thopre]

Landed as c6c4b5dcf1dedd9ac2fab5d405fa2e309b41d156

lnt/server/db/migrations/upgrade_0_to_1.py

 # Version 0 is an empty database.
 #
 # Version 1 is the schema state at the time when we started doing DB
 # versioning.
 
 import sqlalchemy
+from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy import Float, String, Integer, Column, ForeignKey, Binary, DateTime
 from sqlalchemy.orm import relation
-from sqlalchemy.ext.declarative import declarative_base
 
-Base = declarative_base()  # type: sqlalchemy.ext.declarative.api.DeclarativeMeta
+Base = declarative_base()
 
 ###
 # Core Schema
 
 
 class SampleType(Base):
     __tablename__ = 'SampleType'
     id = Column("ID", Integer, primary_key=True)

lnt/server/db/migrations/upgrade_12_to_13.py

 # Adds new table to store jsonschema previously used to construct testsuite.
 import sqlalchemy
+from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy import Column, String, Binary
 
-Base = sqlalchemy.ext.declarative.declarative_base()
+Base = declarative_base()
 
 
 class TestSuiteJSONSchema(Base):
     __tablename__ = "TestSuiteJSONSchemas"
     testsuite_name = Column("TestSuiteName", String(256), primary_key=True)
     jsonschema = Column("JSONSchema", Binary)
 
 
 def upgrade(engine):
     Base.metadata.create_all(engine)

mypy.ini

+[mypy]
+plugins = sqlmypy

tox.ini

commands =
     rm -rf ./test_run_tmp
     lit -sv ./tests/
 
 [testenv:mypy]
 basepython = python3
 skip_install = true
 deps =
     mypy
+    sqlalchemy-stubs
 commands =
     # No where close to passing yet, but nice to have.
     - mypy --junit-xml=junit-{envname}.xml --py2 --ignore-missing-imports lnt
     - mypy --junit-xml=junit-{envname}.xml --ignore-missing-imports lnt
 
 [testenv:flake8]
 skip_install = true
 deps =
     flake8
 
 commands =
     # Still lots of problems.
     - flake8 --statistics --exclude=./lnt/external/  ./lnt/ ./tests/ ./setup.py ./deployment/
     # But lets keep the clean files clean.
     flake8 {[base]whitelist_files}
 
 [testenv:docs]
 skip_install = true
 deps =
     sphinx<2.0.0
     sphinx_bootstrap_theme
 
 commands =
-    make -C {toxinidir}/docs/ html
+    make -C {toxinidir}/docs/ html