libmodbus: new package (#17778)

* libmodbus:adding new package

* fixing testing failures

* fixing flake 8 errors

* fixing flake 8 errors

Co-authored-by: Kapadia <kapadia2@llnl.gov>

var/spack/repos/builtin/packages/libmodbus/package.py

+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class Libmodbus(AutotoolsPackage):
+    """libmodbus is a free software library to send/receive data
+    according to the Modbus protocol.This library is written in C
+    and supports RTU (serial) and TCP (Ethernet) communications."""
+
+    homepage = "https://libmodbus.org/"
+    url      = "https://libmodbus.org/releases/libmodbus-3.0.8.tar.gz"
+
+    version('3.1.6', sha256='d7d9fa94a16edb094e5fdf5d87ae17a0dc3f3e3d687fead81835d9572cf87c16')
+    version('3.1.5', sha256='f7a9538f23a8786b1ee62a4b75879b5c0e194e728350de1b741ce7d595970f06')
+    version('3.1.4', sha256='c8c862b0e9a7ba699a49bc98f62bdffdfafd53a5716c0e162696b4bf108d3637')
+    version('3.1.3', sha256='9e02d79d715522e03b61c313c7278fcf80860816718587819318b8ad9c3fd0ce')
+    version('3.1.2', sha256='661e14f9dc904f3f1b034464ddaa5fd4b8472f8f5d1ea10a1148af85591b7ee9')
+    version('3.1.1', sha256='76d93aff749d6029f81dcf1fb3fd6abe10c9b48d376f3a03a4f41c5197c95c99')
+    version('3.0.8', sha256='022f0691d920b8aee3ee49d7af0f69b7ef80fc3c849a8e0281d5bc27db7a24ea')
+    version('3.0.7', sha256='6c26850cd5dedcf5dad40977ac7f2ee990a3667f6959a1e05e22959bdf537961')
+    version('3.0.6', sha256='046d63f10f755e2160dc56ef681e5f5ad3862a57c1955fd82e0ce036b69471b6')
+    version('3.0.5', sha256='19aad5d55fa315602d6e836a858a3802f1608f9d824afba05fa12a58a1b1e656')