Repository: huggingface/transformers

Issue:
"setup.py" does not seem to have been updated for v3.5.1
## Environment info

I try 
```
!git clone https://github.com/huggingface/transformers.git
%cd transformers/
!pip install -e .
```
on Colaboratory.
     
- `transformers` version: 3.5.0   <- this seems strange.
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.7.0+cu101 (True)
- Tensorflow version (GPU?): 2.3.0 (True)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

### Who can help
 
 documentation: @sgugger

## Information

"setup.py" does not seem to have been updated for v3.5.1.

When I install transformers by `pip install -e .`, the version of transformers is shown as v3.5.0.

## To reproduce

Steps to reproduce the behavior:

I try 
```
!git clone https://github.com/huggingface/transformers.git
%cd transformers/
!pip install -e .
```
on Colaboratory after v3.5.1 release.

Then,
```
import transformers
transformers.__version__
```
returns 
```
'3.5.0'
```

## Expected behavior

The return of `transformers.__version__` is expected to be '3.5.1' now, if my understanding is not wrong.

Maybe, in  https://github.com/huggingface/transformers/blob/afb50c663a5d5623906ead1e87481926467d59fa/setup.py#L120 

'3.5.0' should be changed to '3.5.1'.

Is my understanding correct? Sorry if I misunderstand your intension.



Discussion:
I found https://github.com/huggingface/transformers/commit/d5b3e56de5376aa85ef46e7f0325139d9e299a41 and it seems the related files are updated there.
If there is a reason (or rules for releases) not to merge the change into the master branch, I'm sorry for opening this issue that I haven't fully considered.
This was a hotfix on a branch which is why we didn't update master (mostly because we forgot).
To do things right, we'll actually put v4.0.0-dev since that's what master is right now :-)

Resolving patch:
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -42,6 +42,8 @@
 8. Add the release version to docs/source/_static/js/custom.js and .circleci/deploy.sh
 
 9. Update README.md to redirect to correct documentation.
+
+10. Update the version in __init__.py, setup.py to the new version "-dev" and push to master.
 """
 
 import os
@@ -117,7 +119,7 @@
 
 setup(
     name="transformers",
-    version="3.5.0",
+    version="4.0.0-dev",
     author="Thomas Wolf, Lysandre Debut, Victor Sanh, Julien Chaumond, Sam Shleifer, Patrick von Platen, Sylvain Gugger, Google AI Language Team Authors, Open AI team Authors, Facebook AI Authors, Carnegie Mellon University Authors",
     author_email="thomas@huggingface.co",
     description="State-of-the-art Natural Language Processing for TensorFlow 2.0 and PyTorch",
diff --git a/src/transformers/__init__.py b/src/transformers/__init__.py
--- a/src/transformers/__init__.py
+++ b/src/transformers/__init__.py
@@ -2,7 +2,7 @@
 # There's no way to ignore "F401 '...' imported but unused" warnings in this
 # module, but to preserve other warnings. So, don't check this module at all.
 
-__version__ = "3.5.0"
+__version__ = "4.0.0-dev"
 
 # Work around to update TensorFlow's absl.logging threshold which alters the
 # default Python logging output behavior when present.


Test patch: