Use spawn launch method in asv

In default method asv will close stdin and new pytools looks at stdin
to determine if the session is interactive.

Solution found in pandas-dev/pandas#25237

build-and-benchmark-py-project.sh

@@ -37,8 +37,8 @@ fi
 master_commit=`git rev-parse master`
 test_commit=`git rev-parse HEAD`
 
-asv run $master_commit...$master_commit~ --skip-existing --verbose --show-stderr
-asv run $test_commit...$test_commit~ --skip-existing --verbose --show-stderr
+asv run $master_commit...$master_commit~ --skip-existing --verbose --show-stderr --launch-method=spawn
+asv run $test_commit...$test_commit~ --skip-existing --verbose --show-stderr --launch-method=spawn
 
 output=`asv compare $master_commit $test_commit --factor 1 -s`
 echo "$output"