Add a check for Python 2.7 in build/dev.sh.

Addresses issue #97, e.g. for Arch Linux.

build/dev.sh

@@ -16,6 +16,23 @@ set -o errexit

source test/common.sh  # R_PATH

# In some distros, 'python' is python3, which confuses first-time developers.
# https://github.com/oilshell/oil/issues/97
readonly PYTHON_VERSION="$(python --version 2>&1)"

# bash weirdness: VERSION_REGEX must NOT be inline in the expression, and must
# NOT be quoted.
readonly VERSION_REGEX='Python (2\.7\.[0-9]+)'

if [[ "${PYTHON_VERSION}" =~ $VERSION_REGEX ]]; then
  true
else
  echo >&2 'FATAL: Oil dev requires Python 2.7.*'
  echo >&2 "But you have '${PYTHON_VERSION}'"
  echo >&2 'Hint: Use virtualenv to create a Python 2.7 environment.'
  exit 1
fi

ubuntu-deps() {
  # python-dev: for pylibc
  # gawk: used by spec-runner.sh for the special match() function.