[ty] Add version hint for failed stdlib attribute accesses (#20909)

This is the ultra-minimal implementation of

* https://github.com/astral-sh/ty/issues/296

that was previously discussed as a good starting point. In particular we
don't actually bother trying to figure out the exact python versions,
but we still mention "hey btw for No Reason At All... you're on python
3.10" when you try to access something that has a definition rooted in
the stdlib that we believe exists sometimes.

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -2558,6 +2558,28 @@ class C:
 reveal_type(C().x)  # revealed: Unknown | tuple[Divergent, Literal[1]]
 ```
 
+## Attributes of standard library modules that aren't yet defined
+
+For attributes of stdlib modules that exist in future versions, we can give better diagnostics.
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+`main.py`:
+
+```py
+import datetime
+
+# error: [unresolved-attribute]
+reveal_type(datetime.UTC)  # revealed: Unknown
+# error: [unresolved-attribute]
+reveal_type(datetime.fakenotreal)  # revealed: Unknown
+```
+
 ## References
 
 Some of the tests in the *Class and instance variables* section draw inspiration from

crates/ty_python_semantic/resources/mdtest/snapshots/attributes.md_-_Attributes_-_Attributes_of_standa…_(49ba2c9016d64653).snap

@@ -0,0 +1,51 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: attributes.md - Attributes - Attributes of standard library modules that aren't yet defined
+mdtest path: crates/ty_python_semantic/resources/mdtest/attributes.md
+---
+
+# Python source files
+
+## main.py
+
+```
+1 | import datetime
+2 | 
+3 | # error: [unresolved-attribute]
+4 | reveal_type(datetime.UTC)  # revealed: Unknown
+5 | # error: [unresolved-attribute]
+6 | reveal_type(datetime.fakenotreal)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error[unresolved-attribute]: Type `<module 'datetime'>` has no attribute `UTC`
+ --> src/main.py:4:13
+  |
+3 | # error: [unresolved-attribute]
+4 | reveal_type(datetime.UTC)  # revealed: Unknown
+  |             ^^^^^^^^^^^^
+5 | # error: [unresolved-attribute]
+6 | reveal_type(datetime.fakenotreal)  # revealed: Unknown
+  |
+info: Python 3.10 was assumed when accessing `UTC` because it was specified on the command line
+info: rule `unresolved-attribute` is enabled by default
+
+```
+
+```
+error[unresolved-attribute]: Type `<module 'datetime'>` has no attribute `fakenotreal`
+ --> src/main.py:6:13
+  |
+4 | reveal_type(datetime.UTC)  # revealed: Unknown
+5 | # error: [unresolved-attribute]
+6 | reveal_type(datetime.fakenotreal)  # revealed: Unknown
+  |             ^^^^^^^^^^^^^^^^^^^^
+  |
+info: rule `unresolved-attribute` is enabled by default
+
+```