[Python] Deprecate pyarrow.open_stream,open_file in favor of pa.ipc.open_stream/open_file

[asfimport]

In interest of disambiguating different meanings of "streams" and "files" I propose that we change these APIs to come out of the pyarrow.ipc namespace. To help with the transition, we can maintain functions in the top level namespace that create a FutureWarning. Since a lot of people use these functions, we should probably give them 2 major releases to get migrated

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #3244

_{Note: This issue was originally created as ARROW-4098. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 3244
#3244