Bug 1764711: Move boto3 to requirement file r=ahal

The `upload_geneated_sources.py` script needs `boto3`, but was:
* Running in CI with Mach having a site-packages-source of `None`
* Using the `common` site (which inherits Mach's site-packages-source)
* Installing `boto3` outside of the knowledge of the centralized
  dependency system.

Make a `upload-generated-sources` site, and move `boto3` to it.

Differential Revision: https://phabricator.services.mozilla.com/D143715

build/upload_generated_sources.py

@@ -10,7 +10,6 @@ from contextlib import contextmanager
 import gzip
 import io
 import logging
-from mozbuild.base import MozbuildObject
 from mozbuild.generated_sources import (
     get_filename_with_digest,
     get_s3_region_and_bucket,
@@ -161,10 +160,6 @@ def main(argv):
     args = parser.parse_args(argv)
     region, bucket = get_s3_region_and_bucket()
 
-    config = MozbuildObject.from_environment()
-    config.activate_virtualenv()
-    config.virtualenv_manager.install_pip_package("boto3==1.4.4")
-
     with timed() as elapsed:
         do_work(region=region, bucket=bucket, artifact=args.artifact)
         log.info("Finished in {:.03f}s".format(elapsed()))

python/sites/upload-generated-sources.txt

@@ -0,0 +1 @@
+pypi:boto3==1.4.4

taskcluster/ci/upload-generated-sources/kind.yml

@@ -32,7 +32,7 @@ job-template:
         max-run-time: 600
     run:
         using: mach
-        mach: {artifact-reference: "python build/upload_generated_sources.py <build/public/build/target.generated-files.tar.gz>"}
+        mach: {artifact-reference: "python --virtualenv upload-generated-sources build/upload_generated_sources.py <build/public/build/target.generated-files.tar.gz>"}
         sparse-profile: upload-generated-sources
     scopes:
         - secrets:get:project/releng/gecko/build/level-{level}/gecko-generated-sources-upload