Setup mypy in `tox -e typing` and get it to pass

This is the smallest possible change to get mypy passing on the
jsonschema codebase. The goal of this configuration is to enforce type
annotations anywhere that they appear.
That is, if a method is added to the codebase,

    def foo(x: int) -> str:
        return str(x)

then usages of `foo` will by type checked. If no annotations are
added, `mypy` will not type check functions.

For the most part, this keeps the impact low. The one exceptional case
is the use of `pyrsistent.pmap` as an argument to
`attr.ib(converter=...)`. Unfortunately, it causes `mypy` to
incorrectly deduce the type of the init parameter created by attrs. We
need to "explain the type of init" to mypy by creating a callable with
a concrete type to act as the converter. The callable in question
simply wraps `pmap` with a cast and presents the desired type
information to mypy.

1 files changed, 10 insertions, 0 deletions

diff --git a/tox.ini b/tox.ini
index eea61c6..ced13cc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,6 +5,7 @@ envlist =
     safety
     secrets
     style
+    typing
     docs-{html,doctest,linkcheck,spelling,style}
 skipsdist = True
 
@@ -89,6 +90,15 @@ deps =
 commands =
     {envpython} -m flake8 {posargs} {toxinidir}/jsonschema {toxinidir}/docs
 
+[testenv:typing]
+skip_install = true
+deps =
+    mypy
+    pyrsistent
+    types-attrs
+    types-requests
+commands = {envpython} -m mypy --config {toxinidir}/setup.cfg {posargs} {toxinidir}/jsonschema
+
 [testenv:docs-dirhtml]
 commands = {envpython} -m sphinx -b dirhtml {toxinidir}/docs/ {envtmpdir}/build {posargs:-a -n -q -T -W}
 deps =