Guard typing imports

[chlowell]

typing isn't in azure-core's install_requires and isn't part of 2.7's standard library, so the first from typing import ... can raise on 2.7. Fix would be to either take the dependency on typing or guard imports e.g.:

try:
    from typing import TYPE_CHECKING
except ImportError:
    TYPE_CHECKING = False

if TYPE_CHECKING:
    # pylint:disable=unused-import
    from typing import Any, Mapping, Optional, Type

[mayurid]

Azure core already introduced this dependency. Per the conversation with @johanste each library should have its own dependency (aka not reply on core to bring it)

[lmazuel]

Actually as of today on the repo all packages have a dependency on typing already :)