pyproject: Fix for Python 3.10

Here's what the poetry solver has to say:

---
The current project's Python requirement (3.10.0rc1) is not compatible
with some of the required packages Python requirement:
    - dataclasses requires Python >=3.6, <3.7, so it will not be
    - satisfied for Python 3.10.0rc1

Because dataclasses (0.8) requires Python >=3.6, <3.7
 and no versions of dataclasses match >=0.6,<0.8 || >0.8, dataclasses
 is forbidden.

So, because cli-ui depends on black (21.6b0) which depends on
dataclasses (>=0.6), version solving failed.
---

Author: dmerejkowsky

poetry.lock

@@ -32,7 +32,7 @@ pytest = "^6.2.1"
 pytest-cov = "^2.10"
 
 # Linters
-black = { version = "21.6b0", python = ">=3.6.2" }
+black = { version = "21.7b0", python = ">=3.7" }
 flake8 = "3.9.2"
 flake8-bugbear = "^21.4.3"
 flake8-comprehensions = "^3.4.0"