don't require prompt toolkit for docs on user side

we'll make sure it's available on CI when the docs are generated for the
website, but it shouldn't fail to build in userspace just because of the
ptk events hack thing

docs/conf.py

@@ -11,6 +11,7 @@ import os
 import sys
 import builtins
 import inspect
+import importlib
 
 os.environ['XONSH_DEBUG'] = '1'
 
@@ -20,9 +21,13 @@ from xonsh.xontribs import xontrib_metadata
 from xonsh import main
 from xonsh.commands_cache import CommandsCache
 
-# hacky runaround to import PTK-specific events
-builtins.__xonsh_env__ = Env()
-from xonsh.ptk.shell import events
+spec = importlib.util.find_spec('prompt_toolkit')
+if spec is not None:
+    # hacky runaround to import PTK-specific events
+    builtins.__xonsh_env__ = Env()
+    from xonsh.ptk.shell import events
+else:
+    from xonsh.events import events
 
 sys.path.insert(0, os.path.dirname(__file__))