gh-72327: Suggest using system terminal for pip install in PyREPL (#136328)

Users new to Python packaging often try to use pip from the REPL only to
be met with a confusing SyntaxError. If this happens, guide the user to
use a system terminal instead to invoke pip.

Closes #72327

---------

Co-authored-by: Tom Viner <tom@viner.tv>
Co-authored-by: Brian Schubert <brianm.schubert@gmail.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>

Lib/_pyrepl/console.py

@@ -27,6 +27,7 @@
 import linecache
 from dataclasses import dataclass, field
 import os.path
+import re
 import sys
 
 
@@ -195,7 +196,19 @@ def runsource(self, source, filename="<input>", symbol="single"):
                 ast.PyCF_ONLY_AST,
                 incomplete_input=False,
             )
-        except (SyntaxError, OverflowError, ValueError):
+        except SyntaxError as e:
+            # If it looks like pip install was entered (a common beginner
+            # mistake), provide a hint to use the system command prompt.
+            if re.match(r"^\s*(pip3?|py(thon3?)? -m pip) install.*", source):
+                e.add_note(
+                    "The Python package manager (pip) can only be used"
+                    " outside of the Python REPL.\n"
+                    "Try the 'pip' command in a separate terminal or"
+                    " command prompt."
+                )
+            self.showsyntaxerror(filename, source=source)
+            return False
+        except (OverflowError, ValueError):
             self.showsyntaxerror(filename, source=source)
             return False
         if tree.body: