refactor(taps): Change SQLStream.schema into a cached property (#1745)

* Change schema into a cached_property

Schema is called at least once per record and does not change within a stream, so it should be cached for performance benefits.

* Fix missing sys import

* Changes for 3.7 compatibility

```
@Property
@lru_cache()
```

is a backwards compatible version of functools.cached_property for Python 3.7

https://stackoverflow.com/questions/4037481/caching-class-attributes-in-python

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* Add parentheses to lru_cache for py 3.7

* Fix schema return type

mypy checks fail because the schema return type is lru_cache:

```
singer_sdk/streams/sql.py:82: error: Signature of "schema" incompatible with supertype "Stream"  [override]
singer_sdk/streams/sql.py:154: error: Argument "schema" to "get_selected_schema" has incompatible type "_lru_cache_wrapper[Dict[Any, Any]]"; expected "Dict[Any, Any]"  [arg-type]
Found 2 errors in 1 file (checked 59 source files)
```

The update should pass over the `_lru_cache_wrapper` type and return a `Dict` as expected

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* Fix for mypy

- Remove `lru_cache`
- Set `_cached_schema` up as a class variable to hold the schema for each stream
- Check for presence of `self._cached_schema` on the stream

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* Re-add removed TYPE_CHECKING

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Edgar R. M <edgar@meltano.com>

singer_sdk/streams/sql.py

@@ -20,6 +20,7 @@ class SQLStream(Stream, metaclass=abc.ABCMeta):
     """Base class for SQLAlchemy-based streams."""
 
     connector_class = SQLConnector
+    _cached_schema: dict | None = None
 
     def __init__(
         self,
@@ -74,7 +75,7 @@ def metadata(self) -> MetadataMapping:
         """
         return self._singer_catalog_entry.metadata
 
-    @property
+    @property  # TODO: Investigate @cached_property after py > 3.7
     def schema(self) -> dict:
         """Return metadata object (dict) as specified in the Singer spec.
 
@@ -83,7 +84,13 @@ def schema(self) -> dict:
         Returns:
             The schema object.
         """
-        return t.cast(dict, self._singer_catalog_entry.schema.to_dict())
+        if not self._cached_schema:
+            self._cached_schema = t.cast(
+                dict,
+                self._singer_catalog_entry.schema.to_dict(),
+            )
+
+        return self._cached_schema
 
     @property
     def tap_stream_id(self) -> str: