Stop hardcoding macos version for libtorch wheel builds

[ezyang]

I don't want to have to issue commits like this:

commit c12fbb2d493613c263a0cb189debf06f9504127b (HEAD -> master, origin/master, origin/HEAD)
Author: Edward Z. Yang <ezyang@fb.com>
Date:   Wed Aug 14 10:05:01 2019 -0400

    Fix 3.7 wheel build too
    
    Signed-off-by: Edward Z. Yang <ezyang@fb.com>

diff --git a/wheel/build_wheel.sh b/wheel/build_wheel.sh
index 0d936c6..cbc1aae 100755
--- a/wheel/build_wheel.sh
+++ b/wheel/build_wheel.sh
@@ -97,7 +97,9 @@ mkdir -p "$whl_tmp_dir"
 # update this!
 # An example of this happened on Aug 13, 2019, when osx-64/python-2.7.16-h97142e2_2.tar.bz2
 # was uploaded to https://anaconda.org/anaconda/python/files
-if [[ "$desired_python" == 3.5 ]]; then
+if [[ "$desired_python" == 3.7 ]]; then
+    mac_version='macosx_10_9_x86_64'
+elif [[ "$desired_python" == 3.5 ]]; then
     mac_version='macosx_10_6_x86_64'
 else
     mac_version='macosx_10_7_x86_64'