pgkdep/freebsd: Use py37-pycodestyle

py27-pycodestyle was dropped and is no longer available for the
amd64 arch. Use py37-pycodestyle instead (it should be available for
other common architectures too).

Change-Id: Icb56860d358e2fa06f202a47302a5005b32708e3
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/5877


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Community-CI: Mellanox Build Bot