Add rudimentary mypy type checking

Add a very lax mypy configuration, add it to tox -e linting, and
fix/ignore the few errors that come up. The idea is to get it running
before diving in too much.

This enables:

- Progressively adding type annotations and enabling more strict
  options, which will improve the codebase (IMO).

- Annotating the public API in-line, and eventually exposing it to
  library users who use type checkers (with a py.typed file).

Though, none of this is done yet.

Refs https://github.com/pytest-dev/pytest/issues/3342.

src/_pytest/python_api.py

@@ -9,6 +9,7 @@ from collections.abc import Sized
 from decimal import Decimal
 from itertools import filterfalse
 from numbers import Number
+from typing import Union
 
 from more_itertools.more import always_iterable
 
@@ -58,7 +59,8 @@ class ApproxBase:
             a == self._approx_scalar(x) for a, x in self._yield_comparisons(actual)
         )
 
-    __hash__ = None
+    # Ignore type because of https://github.com/python/mypy/issues/4266.
+    __hash__ = None  # type: ignore
 
     def __ne__(self, actual):
         return not (actual == self)
@@ -202,8 +204,10 @@ class ApproxScalar(ApproxBase):
     Perform approximate comparisons where the expected value is a single number.
     """
 
-    DEFAULT_ABSOLUTE_TOLERANCE = 1e-12
-    DEFAULT_RELATIVE_TOLERANCE = 1e-6
+    # Using Real should be better than this Union, but not possible yet:
+    # https://github.com/python/typeshed/pull/3108
+    DEFAULT_ABSOLUTE_TOLERANCE = 1e-12  # type: Union[float, Decimal]
+    DEFAULT_RELATIVE_TOLERANCE = 1e-6  # type: Union[float, Decimal]
 
     def __repr__(self):
         """
@@ -261,7 +265,8 @@ class ApproxScalar(ApproxBase):
         # Return true if the two numbers are within the tolerance.
         return abs(self.expected - actual) <= self.tolerance
 
-    __hash__ = None
+    # Ignore type because of https://github.com/python/mypy/issues/4266.
+    __hash__ = None  # type: ignore
 
     @property
     def tolerance(self):
@@ -691,7 +696,7 @@ def raises(expected_exception, *args, **kwargs):
     fail(message)
 
 
-raises.Exception = fail.Exception
+raises.Exception = fail.Exception  # type: ignore
 
 
 class RaisesContext: