Add seekable() to mmap.mmap (#8405)

* Allow mmap-backed readers to identify seek support

mmap already supports seek() and tell(); expose seekable() to match CPython.

Confidence: high
Scope-risk: narrow
Tested: test_mmap, stdlib_mmap snippet, clippy, prek, extra_tests
Not-tested: full workspace test blocked by local rustpython-capi SIGSEGV
Assisted-by: Codex:gpt-5.6-sol

* Keep mmap regression test lint-clean

CI ruff check removes the redundant blank line and treats the auto-fix as a failure.

Constraint: CI runs prek with ruff check
Confidence: high
Scope-risk: narrow
Tested: prek run --all-files; cargo run -- extra_tests/snippets/stdlib_mmap.py
Assisted-by: Codex:gpt-5.6-sol

* Keep mmap seekability method conventional

The method is invoked at runtime through the Python wrapper, so const qualification provides no benefit.

Constraint: Python methods execute at runtime
Confidence: high
Scope-risk: narrow
Tested: prek run --all-files; cargo run -- extra_tests/snippets/stdlib_mmap.py
Assisted-by: Codex:gpt-5.6-sol

extra_tests/snippets/stdlib_mmap.py

@@ -0,0 +1,6 @@
+import mmap
+
+mapped = mmap.mmap(-1, 1)
+assert mapped.seekable()
+mapped.close()
+assert mapped.seekable()