Remove boto3 dependency
Downloading a model from `s3://` urls was not documented anywhere and I suspect it doesn't work with our `from_pretrained` methods anyways.

Removing boto3 also kills its transitive dependencies (some of which are slow to support new versions of Python), contributing to a leaner library:

```
boto3-1.12.46
botocore-1.15.46
docutils-0.15.2
jmespath-0.9.5
python-dateutil-2.8.1
s3transfer-0.3.3
six-1.14.0
urllib3-1.25.9
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=h1) Report
> Merging [#3968](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/4e817ff41885063e08bb3bcd63e5adfd835b9911&el=desc) will **increase** coverage by `0.07%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3968/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3968      +/-   ##
==========================================
+ Coverage   78.44%   78.51%   +0.07%     
==========================================
  Files         111      111              
  Lines       18518    18486      -32     
==========================================
- Hits        14527    14515      -12     
+ Misses       3991     3971      -20     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3968/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `72.61% <100.00%> (+3.74%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3968/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.06% <0.00%> (+0.12%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=footer). Last update [4e817ff...b440a19](https://codecov.io/gh/huggingface/transformers/pull/3968?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
