package/python-cryptography: bump version to 2.3.1

package/python-cryptography: bump version to 2.3.1

Changelog: https://cryptography.io/en/latest/changelog/#v2-3-1

Please note that CVE-2018-10903, fixed in version 2.3, was introduced
in version 1.9.0, so it was not present in buildroot:
https://nvd.nist.gov/vuln/detail/CVE-2018-10903

Added license hashes and switched runtime dependency from pyasn1 to
asn1crypto: https://cryptography.io/en/latest/changelog/#v1-8

Fixed pthread problem by adding -pthread to CFLAGS, suggested by
https://patchwork.openembedded.org/patch/146240/
https://bugs.gentoo.org/630578#c6

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>