Speedup check_copies script
Checking the copies by using black was slowing down the script quite a lot, so removing this check makes the script way faster. Removing `blackify` use could make the script less robust though, so leaving the function for now even if we don't use it anymore. If a situation arises where we see the script fail, I can code a (more complex) way of using black that would be fast.

With the two lines removed, the script takes 0.129s on my setup (instead of 18s).

cc @stas00 for information.
Whoah! That's blazing fast! Thanks, @sgugger!

I think that's why `flake8` is slow - it runs black in some slow mode (`black` itself is very fast)

You can always add a flag that activates that disabled function, so it's there if needed.<|||||>This is no longer needed: `--line-length 119 --target-version py35` at https://github.com/huggingface/transformers/blob/90d1545f25b02a05b1581ae7a617db609fece0a0/utils/check_copies.py#L85
it now uses the config file - ensures we only have one place to do this setting.

Also, I haven't studied your code, but if it's applicable - skip checking files that haven't changes since last check - should give a huge speed increase, since typically only a few files are touched during a development of a single PR. If if is applicable and I can be of help let me know.

I wish black/flake8/isort did that too. It makes no sense to re-run the check on files that haven't changed, which is like 99% of files most of the time.<|||||>No need for a check-since-file-modified approach, use this instead:
```
git diff --name-only $(git merge-base --fork-point master)
```
as the source of what files to check.

It will give you all the files that were modified since the branch was made - yay!

But you only want specific sub-folders, so:

```
git diff --name-only $(git merge-base --fork-point master) | egrep '^(examples|templates|tests|src|utils)' | tr '\n' ' '
```

Now you can unleash whatever checks and it'd be all blazing fast.

I will post shortly a PR to make flake8 and other checkers rocket-fast! https://github.com/huggingface/transformers/pull/7403

I will make a function in Makefile which you can use to feed to the check scripts just the modified files. **edit**:  See https://github.com/huggingface/transformers/pull/7403 you now have a variable with all the modified files.