[Backport] Update the way dockerfiles install pip for python 2.7 and …

…3.5 (grpc#25607)

* Backport: Update the way dockerfiles install pip for python 2.7 and 3.5

* Fix implicit-int-float-conversion warning.

Co-authored-by: Jan Tattermusch <jtattermusch@google.com>
Co-authored-by: Levon Ter-Grigoryan <patrios@google.com>

Relax ruby test check on protobuf error message (grpc#25507) (grpc#25591

)

Co-authored-by: apolcyn <apolcyn@google.com>

Bump version to v1.36.0 (grpc#25534)

Merge pull request grpc#25433 from jtattermusch/backport_25420

Backport grpc#25420 into v1.36.x

Merge pull request grpc#25186 from stanley-cheung/bump-to-1_35

Bump to version 1.35.0

Merge pull request grpc#25135 from yulin-liang/fix_macos_distribtest

[Backport]Fix macos distribtest

Merge pull request grpc#25110 from stanley-cheung/bump-1-35-version

Bump version to v1.35.0-pre1

Merge pull request grpc#24871 from jtattermusch/backport_24831

Backport grpc#24831 to v1.34.x (to fix interop tests build)

Merge pull request grpc#24782 from yashykt/xdssecenvvarbackport

Protect xds security code with the environment variable "GRPC_XDS_EXPERIMENTAL_SECURITY_SUPPORT"

Merge pull request grpc#24583 from donnadionne/release

Bump version to v1.33.2