set encoding utf-8 for reading readme in setup.py

[lorepirri]

Under Windows, while building the wheel:

...
                   INFO     Traceback (most recent call last):                                                                                                                                                      
                   INFO       File "<string>", line 2, in <module>                                                                                                                                                  
                   INFO       File "<pip-setuptools-caller>", line 34, in <module>                                                                                                                                  
                   INFO       File "C:\---\setup.py", line 7, in <module>                                                                           
                   INFO         long_description = (this_directory / "README.md").read_text()                                                                                                                       
                   INFO                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                       
...                                                     
                   INFO         return codecs.charmap_decode(input,self.errors,decoding_table)[0]                                                                                                                   
                   INFO                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                      
                   INFO     UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 115: character maps to <undefined>                                                                               
                   INFO     error: subprocess-exited-with-error                                                                                                                                                     
...

In setup.py the line:

streamlit-vertical-slider/setup.py

Line 7 in 737b6a8

long_description = (this_directory / "README.md").read_text()

is trying to open the README.md file that has at least 1 utf-8 character,

and therefore it is giving an error, for Windows:

UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 115: character maps to <undefined>

You can search for the error line, and see people struggling with that:

• https://stackoverflow.com/questions/9233027/unicodedecodeerror-charmap-codec-cant-decode-byte-x-in-position-y-character

Setting the encoding to utf-8 will fix this for Windows and keep unix platforms working.