Fix azure mailer deployment issue

[h-dub]

Fix for #6012
pkg_resources seems to be unneeded and is causing packaging issues when deploying the azure function

[kapilt]

can your confirm successful e2e message delivery with the change?

tracking back on why pkg_resources was included it looks like it was splunk support in #4044 .. and it looks like azure was working afterwards, but i suspect the function version change in the msi/csi support may have resulted in the issue here.

its also a little unclear why this would fix as afaics pkg_resources never makes it into the requirements file (testing on osx, py 3.8 though)

>>> from c7n_mailer.azure_mailer import deploy
>>> reqs_txt = deploy.get_mailer_requirements()
>>> "pkg_resources" in reqs_txt
False

doing a fresh install in a virtualenv its a little hard to see any explicit dep on pkg_resources in the output. the usages modes between the two providers are a little different though, CORE_DEPS is assembled in aws from local imports, where as in azure we generate a requirements file server side installation and pkg_resources itself comes from setuptools which would always be installed.

a safer solution would be to add it to azure_mailer/deploy generate_requirements ignore.. but given its not in the output of that function its a little more unclear to me why it would resolve.

[h-dub]

Looks like the root cause is a bug between ubuntu and venv that shows as fixed in may
It incorrectly adds pkg_resources==0.0.0.0 on pip freeze
https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1635463

[h-dub]

my guess is the original author of the splunk change was trying to figure out which deps needed to be added and was working from a pip freeze with the bug. this matches the vague comment on the pkg_resources dep being added.

we should have never needed pkg_resources as a listed dep
i will validate its working in aws without pkg_resources:
if it works in aws, we can take that fix
if it fails in aws, I have a second working fix that just excludes it from azure function packaging

[h-dub]

in aws lambda, it works because there is a pkg_resources=0.0.0 available as a package and it is installed :)
everywhere else, pkg_resources isn't available as a package and should not be in the requirements.txt
I will update this PR with the azure specific fix on the next cycle. We should be able to accept it with minimal risk.

[kapilt]

pkg_resources comes in via setuptools, which isn't in the aws environment, we actually upload clientside by name into the package, but in gcp and azure we do server side building.

[kapilt]

working with hugh i was able to identify why i never hit this on e2e testing because i was using pyenv installed python, which differs significantly from system python installs at least on ubuntu wrt to distutils/setuptools.

[kapilt]

thanks, lgtm