gRFC Proposal: Add Python eventlet support

[Gorka Eguileor]

Hi,

I have submitted a gRFC PR [1] to add a new experimental custom I/O
manager that would allow using the gRPC library in Python applications
that use the eventlet library.

I didn't know about the gRFC process until Richard Belleville mentioned
it, so I submitted a code PR [2] first, and it has also had a first code
review from Lidi Zheng.

One aspect that Richard mentioned was of great importance was the test
stability, so in order to provide some data points in this regard I had
to make some code changes to the `run_tests.py` code that may be of use
to others, so I have submitted a PR for it as well [3].

I would appreciate any comments you may have regarding this gRFC and its
code.

Thanks in advance,
Gorka.

[1]: https://github.com/grpc/proposal/pull/172
[2]: https://github.com/grpc/grpc/pull/22062
[3]: https://github.com/grpc/grpc/pull/22270