Small fix to sage-bootstrap-python

The sage-bootstrap-python script doesn't work with pyenv.
For some reason the command "python" is treated differently
from python, leading to issues. My fix was to try removing
pyenv from the PATH before testing for python.

build/bin/sage-bootstrap-python

@@ -11,7 +11,15 @@ if [ -z "$SAGE_ORIG_PATH" ]; then
     # If not we're running from within sage-env just set the existing path
     SAGE_ORIG_PATH="$PATH"
 fi
-
+paths=$(echo $SAGE_ORIG_PATH | tr ":" "\n")
+NEW_PATH=""
+for path in $paths
+do
+    if [[ $path != *"pyenv"* ]]; then
+        NEW_PATH+="$path:"
+    fi
+done
+SAGE_ORIG_PATH=${NEW_PATH::-1}
 # In particular, it is invoked by "bootstrap -d" for sage-download-file,
 # i.e., before a configure run, and by "sage-spkg", also for sage-download-file.
 # So it needs to find a python that has the urllib module.