pylint: remove g-import-not-at-top suppressions (#3022)

Summary:
This is a Google-specific lint that we kind of just cargo cult around
and don’t actually lint against in CI, which makes it less than useful.

Generated with:

```
git ls-files -z '*.py' |
    xargs -0 sed -i \
        -e 's/g-import-not-at-top,//' \
        -e 's/,g-import-not-at-top//' \
        -e 's/ *# pylint:.*=g-import-not-at-top$//' \
        ;
```

Note: This cherry-pick only includes files under tensorboard/uploader
28 files from the original commit were omitted.

Test Plan:
Our standard lint still passes:

```
$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
0
```

wchargin-branch: unsuppress-nontop-imports

Author: wchargin

tensorboard/uploader/exporter_test.py

@@ -28,9 +28,9 @@
 
 try:
   # python version >= 3.3
-  from unittest import mock  # pylint: disable=g-import-not-at-top
+  from unittest import mock
 except ImportError:
-  import mock  # pylint: disable=g-import-not-at-top,unused-import
+  import mock  # pylint: disable=unused-import
 
 
 from tensorboard.uploader.proto import export_service_pb2

tensorboard/uploader/uploader_test.py

@@ -26,9 +26,9 @@
 
 try:
   # python version >= 3.3
-  from unittest import mock  # pylint: disable=g-import-not-at-top
+  from unittest import mock
 except ImportError:
-  import mock  # pylint: disable=g-import-not-at-top,unused-import
+  import mock  # pylint: disable=unused-import
 
 import tensorflow as tf
 

tensorboard/uploader/util_test.py

@@ -26,9 +26,9 @@
 
 try:
   # python version >= 3.3
-  from unittest import mock  # pylint: disable=g-import-not-at-top
+  from unittest import mock
 except ImportError:
-  import mock  # pylint: disable=g-import-not-at-top,unused-import
+  import mock  # pylint: disable=unused-import
 
 
 from google.protobuf import timestamp_pb2