Release api_core 1.6.0

[crwilcox]

This pull request was generated using releasetool.

[tseaver]

CI failure is for nox -e pytype.

[crwilcox]

@rchen152 is looking into PyType failures a bit. Locally I have some different failures on runs.

crwilcox@redrover ~/w/r/g/api_core> nox -s pytype                                                                                                                                                                    release-api_core-1.6.0
nox > Running session pytype
nox > Creating virtualenv using python3.6 in /Users/crwilcox/workspace/releasetool/google-cloud-python/api_core/.nox/pytype
nox > pip install --upgrade . grpcio >= 1.8.2 grpcio-gcp >= 0.2.2 pytype >= 2018.9.26
nox > pytype
Analyzing 53 sources with 0 dependencies
ninja: Entering directory `/Users/crwilcox/workspace/releasetool/google-cloud-python/api_core/pytype_output'
ninja: error: build.ninja:29: multiple rules generate /Users/crwilcox/workspace/releasetool/google-cloud-python/api_core/pytype_output/pyi/google/api_core/path_template.pyi [-w dupbuild=err]

nox > Command pytype  failed with exit code 1
nox > Session pytype failed.

The error varies each run which is a bit unusual.

ninja: error: build.ninja:44: multiple rules generate /Users/crwilcox/workspace/releasetool/google-cloud-python/api_core/pytype_output/pyi/google/api_core/timeout.pyi [-w dupbuild=err]

ninja: error: build.ninja:47: multiple rules generate /Users/crwilcox/workspace/releasetool/google-cloud-python/api_core/pytype_output/pyi/google/api_core/grpc_helpers.pyi [-w dupbuild=err]

[rchen152]

@crwilcox It looks like what's happening is that pytype is somehow finding multiple files named path_template.py (and timeout.py and grpc_helpers.py - the error probably differs because it varies which file pytype discovers first). The other thing I noticed is that the file count in the status message ("Analyzing 53 sources with 0 dependencies") differs from the one I see (28 sources). Any idea why there might be two copies of some source files?

EDIT: if there are two files with the same name in different directories, this may be google/pytype#198.

[crwilcox]

@rchen152 , I rm -rf'd the output dir (pytype_output) between runs so that is probably related?