add new python and update default

CHANGELOG.md

 # Python Buildpack Changelog
 
-# 151 (2019-03-)
+# 152 (2019-04-02)
+
+Python 3.7.3 now available.
+
+# 151 (2019-03-21)
 
 Python 3.5.7 and 3.4.10 now available on all Heroku stacks.
 

bin/default_pythons

@@ -2,7 +2,7 @@
 
 DEFAULT_PYTHON_VERSION="python-3.6.8"
 LATEST_36="python-3.6.8"
-LATEST_37="python-3.7.2"
+LATEST_37="python-3.7.3"
 LATEST_35="python-3.5.7"
 LATEST_34="python-3.4.10"
 LATEST_27="python-2.7.16"

builds/runtimes/python-3.7.3

+#!/usr/bin/env bash
+# Build Path: /app/.heroku/python/
+
+OUT_PREFIX=$1
+BIN_DIR="$(cd "$(dirname "$0")"/../.. || exit; pwd)/bin"
+export BIN_DIR
+
+# shellcheck source=bin/utils
+source "$BIN_DIR/steps/sqlite3"
+
+sqlite3_version
+echo "Setting up SQLite3 Headers for $SQLITE3_VERSION"
+sqlite3_install "$OUT_PREFIX" "$SQLITE3_VERSION" 1
+
+echo "Building Python…"
+SOURCE_TARBALL='https://python.org/ftp/python/3.7.3/Python-3.7.3.tgz'
+curl -L $SOURCE_TARBALL | tar xz
+mv Python-3.7.3 src
+cd src
+
+./configure --prefix=$OUT_PREFIX --with-ensurepip=no
+make
+make install
+
+# Remove unneeded test directories, similar to the official Docker Python images:
+# https://github.com/docker-library/python
+find "${OUT_PREFIX}" \( -type d -a \( -name test -o -name tests \) \) -exec rm -rf '{}' +
+
+# Remove spare /
+LOCATION=${OUT_PREFIX%?}
+
+ln $LOCATION/bin/python3 $LOCATION/bin/python