pin minimal required deps

pyproject.toml

@@ -44,15 +44,15 @@ classifiers = [
 [tool.poetry.dependencies]
 python = "^3.6"
 
-dataclasses = { version = "*", python = "~3.6" }
+dataclasses = { version = ">=0.6.0", python = "~3.6" }
 
 [tool.poetry.dev-dependencies]
-black = { version = "*", allows-prereleases = true }
-flake8 = "*"
-flake8-bugbear = "*"
-isort = "*"
-msgpack = "*"
-mypy = "*"
-pytest = "*"
-pytest-cov = "*"
-tox = "^3.0.0"
+black = { version = ">=18.9b0", allows-prereleases = true }
+flake8 = ">=3.6.0"
+flake8-bugbear = ">=18.8.0"
+isort = ">=4.3.4"
+msgpack = ">=0.6.0"
+mypy = ">=0.650"
+pytest = ">=4.1.0"
+pytest-cov = ">=2.6.0"
+tox = ">=3.6.0"