ci/deps: pin pylib for testenv on Windows only

Pending fix/investigation via #393.

tox: mypy*: skip deps (i.e. pinned `py` on Windows)

setup.py

@@ -3,7 +3,7 @@
 # TODO: if py gets upgrade to >=1.6,
 #       remove _width_of_current_line in terminal.py
 INSTALL_REQUIRES = [
-    "py>=1.5.0,!=1.8.2,!=1.9.0",
+    "py>=1.5.0",
     "packaging",
     "attrs>=17.4.0",  # should match oldattrs tox env.
     "more-itertools>=4.0.0",

tox.ini

@@ -53,6 +53,8 @@ deps =
     twisted: {env:_PYTEST_TOX_TWISTED:twisted}
     xdist: pytest-xdist>=1.13
     {env:_PYTEST_TOX_EXTRA_DEP:}
+    # Pin pylib for Windows (pending investigation of test failures).
+    py>=1.5.0,!=1.8.2,!=1.9.0 ; sys_platform == 'win32'
 
 [testenv:upstream]
 skip_install = True
@@ -84,11 +86,13 @@ commands =
   pre-commit run --all-files --show-diff-on-failure --config {envtmpdir}/pc.yaml {posargs:}
 
 [testenv:mypy]
+deps =
 extras = checkqa-mypy, testing
 commands = mypy {posargs:src testing}
 
 # Checks default (configured) python_version, and the minimal one.
 [testenv:mypy_ci]
+deps = {[testenv:mypy]deps}
 extras = {[testenv:mypy]extras}
 commands =
   mypy {posargs:src testing}