fix sphinx-rtd-theme version constraint

poetry.lock

@@ -627,7 +627,7 @@ testing = ["pathlib2", "unittest2", "jaraco.itertools", "func-timeout"]
 [metadata]
 lock-version = "1.1"
 python-versions = "~2.7 || ~=3.5"
-content-hash = "8ef35949fb5e1efd6e7d4307c90da48997b80276f92f5dbf5a06ce6145c399b7"
+content-hash = "c3d447a54df484c1355a50d860965f131bc9f62b917ca26911e13c85d51b2776"
 
 [metadata.files]
 alabaster = [

pyproject.toml

@@ -19,8 +19,8 @@ pytest = {version = "~=4.6"}
 mock = "~=3.0"
 mypy = {version = "~=0.800", python = "~=3.5"}
 typing = {version = "~=3.7.4", python = "~=2.7"}
-Sphinx = {version = "^3.5.0", python = "~=3.5"}
-sphinx-rtd-theme = "^0.5.1"
+Sphinx = {version = "~=3.5.0", python = "~=3.5"}
+sphinx-rtd-theme = {version = "~=0.5.1", python = "~=3.5"}
 
 [build-system]
 requires = ["poetry-core>=1.0.0", "Cython", "setuptools"]