Build problem of ujson 4.0.0 in Ubuntu arm64 Docker container

Hello everyone! 



I have a **problem** installing / building ujson in a ubuntu18.04 Docker container for arm64/aarch64. 



This has started with the last change, adding the special characters to setup.py. 

It seems that in setup.py the reading of README.rst has problems with the encoding. 



The **error** I get is 

```

Traceback (most recent call last):

  File "setup.py", line 38, in <module>

    long_description = f.read()

  File "/usr/lib/python3.6/encodings/ascii.py", line 26, in decode

    return codecs.ascii_decode(input, self.errors)[0]

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 2116: ordinal not in range(128)

``` 



A possible **solution** is (not sure what is the best way) to change [Line 37 in setup.py](https://github.com/ultrajson/ultrajson/blob/62b89200c69723bfe1ae3ff56c94a70c99459a9d/setup.py#L37) to `with open("README.rst", "rt", encoding="utf-8") as f:`



To **reproduce** this on a x86 machine (with docker and qemu installed) build the following Dockerfile and start the container: 



```Dockerfile

FROM ubuntu:18.04@sha256:c57d3145ffd3adb9e4dea2704c08d4d027dd57cb6e30d69dce6c1d14bd38fe5c as ujsontest



RUN apt-get update

RUN apt-get -y upgrade

RUN apt -y install git vim build-essential python3 python3-dev python3-pip



RUN git clone https://github.com/ultrajson/ultrajson.git

RUN pip3 install wheel

WORKDIR /ultrajson



RUN pip3 install --upgrade setuptools



ENTRYPOINT python3 setup.py bdist_wheel

```



Run: 

```sh

docker build -t ujsonencodingerror -f Dockerfile .

docker run --rm ujsonencodingerror

```



**Other setups build / install without problems**. (i.e. Ubuntu 18.04 installed on Raspberry Pi 4 (aarch64) and Debian in a x86 VM). 



I would appreciate if someone could look into this. If additional information is necessary, please contact me. 

Thank! 

Cheers,

David



_Originally posted by @DavidBuettner in https://github.com/ultrajson/ultrajson/pull/427#r502500815_

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