prompt: switching to prompt_toolkit in case of stdin with interactive mode (#5536)

### Motivation

There is an edge case that we're using mostly in integration tests from
the beginning of the world:
`echo 'echo 1' | xonsh -i` and it's not working with `TERM=dumb` because
dumb is readline.
So in this case we need to force using prompt_toolkit to avoid hanging.

Closes #5462 #5517

### Before

These all produces hanging because reading stdin is not implemented in
readline backend:
```xsh
echo whoami | TERM=dumb xonsh -i --no-rc   # dumb is readline in fact
# or the same
echo whoami | xonsh -st dumb -i  # dumb is readline in fact
# or the same
echo whoami | xonsh -st readline -i
```

### After

Switching to prompt_toolkit in these cases.

## For community
⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍
comment**

---------

Co-authored-by: a <1@1.1>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

news/prompt_switch_to_ptk.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* prompt: Switching to prompt_toolkit in edge case of sending stdin to interactive mode (#5462 #5517).
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

xonsh/shell.py

@@ -202,9 +202,19 @@ class Shell:
         if is_class(backend):
             cls = backend
         else:
+            """
+            There is an edge case that we're using mostly in integration tests:
+            `echo 'echo 1' | xonsh -i` and it's not working with `TERM=dumb` (#5462 #5517)
+            because `dumb` is readline where stdin is not supported yet. PR is very welcome!
+            So in this case we need to force using prompt_toolkit.
+            """
+            is_stdin_to_interactive = (
+                XSH.env.get("XONSH_INTERACTIVE", False) and not sys.stdin.isatty()
+            )
+
             if backend == "none":
                 from xonsh.base_shell import BaseShell as cls
-            elif backend == "prompt_toolkit":
+            elif backend == "prompt_toolkit" or is_stdin_to_interactive:
                 from xonsh.ptk_shell.shell import PromptToolkitShell as cls
             elif backend == "readline":
                 from xonsh.readline_shell import ReadlineShell as cls