I'm trying to debug a python codebase that uses tox for unit tests. One of the failing tests is proving difficult due to figure out, and I'd like to use pudb to step through the code. 

At first thought, one would think to just `pip install pudb` then in the unit test code add in `import pudb` and `pudb.settrace()`. But that results in a ModuleNotFoundError:

```
>       import pudb
>E       ModuleNotFoundError: No module named 'pudb'
>tests/mytest.py:130: ModuleNotFoundError
> ERROR: InvocationError for command '/Users/me/myproject/.tox/py3/bin/pytest tests' (exited with code 1)

```

Noticing the .tox project folder leads one to realize there's a site-packages folder within tox, which makes sense since the point of tox is to manage testing under different virtualenv scenarios. This also means there's a tox.ini configuration file, with a deps section that may look like this:

```
[tox]
envlist = lint, py3

[testenv]
deps =
    pytest
commands = pytest tests

```

adding `pudb` to the deps list should solve the ModuleNotFoundError, but leads to another error:

```
self = <_pytest.capture.DontReadFromInput object at 0x103bd2b00>

    def fileno(self):
>       raise UnsupportedOperation("redirected stdin is pseudofile, "
                                   "has no fileno()")
E       io.UnsupportedOperation: redirected stdin is pseudofile, has no fileno()

.tox/py3/lib/python3.6/site-packages/_pytest/capture.py:583: UnsupportedOperation

```

So, I'm stuck at this point. Is it not possible to use pudb instead of pdb within Tox?