Prepare upload scripts for Python 3.7 removal (#479)

As we will remove Python 3.7, we need to update the Python version in the upload scripts

Authors:
  - Jordan Jacobelli (https://github.com/Ethyling)

Approvers:
  - Sevag Hanssian (https://github.com/sevagh)
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: #479

ci/cpu/prebuild.sh

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 
 DEFAULT_CUDA_VER="11.5"
+DEFAULT_PYTHON_VER="3.8"
 
 #Upload cuspatial once per PYTHON
 if [[ "$CUDA" == "${DEFAULT_CUDA_VER}" ]]; then
@@ -10,7 +11,7 @@ else
 fi
 
 #Upload libcuspatial once per CUDA
-if [[ "$PYTHON" == "3.7" ]]; then
+if [[ "$PYTHON" == "${DEFAULT_PYTHON_VER}" ]]; then
     export UPLOAD_LIBCUSPATIAL=1
 else
     export UPLOAD_LIBCUSPATIAL=0