Update pyserial requirement from ~=3.4.0 to ~=3.5

Updates the requirements on [pyserial](https://github.com/pyserial/pyserial) to permit the latest version.
- [Release notes](https://github.com/pyserial/pyserial/releases)
- [Changelog](https://github.com/pyserial/pyserial/blob/master/CHANGES.rst)
- [Commits](pyserial/pyserial@v3.4...v3.5)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,7 +1,7 @@
 hjson~=3.0.1
 mock~=3.0.5
 pyroute2==0.5.*
-pyserial~=3.4.0
+pyserial~=3.5
 python-pppd==1.0.3
 python-sdk-auth~=0.3.0
 pyudev~=0.22.0