Add extra version of py-sqlalchemy (#15109)

* Add extra version of py-sqlalchemy

* Update package.py

* Update package.py

* Update package.py

* Update package.py

* Update package.py

* Apply suggestions from code review

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Update package.py

* Update package.py

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/py-sqlalchemy/package.py

@@ -14,6 +14,7 @@ class PySqlalchemy(PythonPackage):
 
     version('1.3.9', sha256='272a835758908412e75e87f75dd0179a51422715c125ce42109632910526b1fd')
     version('1.2.19', sha256='5bb2c4fc2bcc3447ad45716c66581eab982c007dcf925482498d8733f86f17c7')
+    version('1.2.10', sha256='72325e67fb85f6e9ad304c603d83626d1df684fdf0c7ab1f0352e71feeab69d8')
     version('1.1.18', sha256='8b0ec71af9291191ba83a91c03d157b19ab3e7119e27da97932a4773a3f664a9')
     version('1.0.12', sha256='6679e20eae780b67ba136a4a76f83bb264debaac2542beefe02069d0206518d1')
 
@@ -22,3 +23,10 @@ class PySqlalchemy(PythonPackage):
     depends_on('py-pytest@2.5.2:3.9.0,3.9.3:', type='test')
     depends_on('py-mock', type='test')
     depends_on('py-pytest-xdist', type='test')
+
+    variant('backend', description='Python modules for database access',
+            values=any_combination_of('mysql', 'pymysql', 'postgresql'))
+
+    depends_on('py-mysqlclient', when='backend=mysql',      type=('build', 'run'))
+    depends_on('py-pymysql',     when='backend=pymysql',    type=('build', 'run'))
+    depends_on('py-psycopg2',    when='backend=postgresql', type=('build', 'run'))