Improve bert-japanese tokenizer handling (#8659)

* Make ci fail

* Try to make tests actually run?

* CI finally failing?

* Fix CI

* Revert "Fix CI"

This reverts commit ca7923be7334d4e571b023478ebdd6b33dfd0ebb.

* Ooops wrong one

* one more try

* Ok ok let's move this elsewhere

* Alternative to globals() (#8667)

* Alternative to globals()

* Error is raised later so return None

* Sentencepiece not installed make some tokenizers None

* Apply Lysandre wisdom

* Slightly clearer comment?

cc @sgugger

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

.circleci/config.yml

@@ -221,7 +221,7 @@ jobs:
     run_tests_custom_tokenizers:
         working_directory: ~/transformers
         docker:
-            - image: circleci/python:3.6
+            - image: circleci/python:3.7
         environment:
             RUN_CUSTOM_TOKENIZERS: yes
         steps: