Make python a runtime dependency in conda's meta.yml

Analogue of cornellius-gp/gpytorch#2457, necesary b/c of conda/conda#13412

Author: Balandat

.conda/meta.yaml

@@ -17,6 +17,7 @@ requirements:
     - setuptools
     - setuptools_scm
   run:
+    - python >=3.9
     - pytorch >=1.13.1
     - gpytorch ==1.11
     - linear_operator ==0.5.1