chore(deps): update dependency sqlalchemy to v1.3.15 (#3201)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [SQLAlchemy](http://www.sqlalchemy.org) | patch | `==1.3.13` -> `==1.3.15` |

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appengine/standard_python37/cloudsql/requirements.txt

@@ -2,4 +2,4 @@ flask==1.1.1
 # psycopg2==2.8.4
 psycopg2-binary==2.8.4 # you will need either the binary or the regular - for more info see http://initd.org/psycopg/docs/install.html
 PyMySQL==0.9.3
-SQLAlchemy==1.3.13
+SQLAlchemy==1.3.15

cloud-sql/mysql/sqlalchemy/requirements.txt

@@ -1,3 +1,3 @@
 Flask==1.1.1
-SQLAlchemy==1.3.13
+SQLAlchemy==1.3.15
 PyMySQL==0.9.3

cloud-sql/postgres/sqlalchemy/requirements.txt

@@ -1,4 +1,4 @@
 Flask==1.1.1
-SQLAlchemy==1.3.13
+SQLAlchemy==1.3.15
 pg8000==1.13.2; python_version > '3.0'
 pg8000==1.12.5; python_version < '3.0'