use isolated_build in tox.ini

The isolated_build option is required when pyproject.toml is present.
Add it to fix the following tox error:

```
ERROR: pyproject.toml file found.
To use a PEP 517 build-backend you are required to configure tox to use an isolated_build:
https://tox.readthedocs.io/en/latest/example/package.html
```

Author: mgorny

tox.ini

@@ -1,5 +1,6 @@
 [tox]
 envlist = update, py36, py37, py38, py39, py310, mypy-py36
+isolated_build = True
 
 [testenv]
 deps = pytest