docs: simplify install instructions
docs: simplify install instructions

We only need pip3 now on all OSes as cryptography now ships wheels on Linux :tada:.
Also remove `pip3 install --user` recommendation because that won't end up on $PATH.

StevenVanAcker/mitmproxy,MatthewShao/mitmproxy,cortesi/mitmproxy,MatthewShao/mitmproxy,mitmproxy/mitmproxy,zlorb/mitmproxy,mhils/mitmproxy,zlorb/mitmproxy,mitmproxy/mitmproxy,cortesi/mitmproxy,StevenVanAcker/mitmproxy,mitmproxy/mitmproxy,mitmproxy/mitmproxy,zlorb/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy,ddworken/mitmproxy,vhaupert/mitmproxy,ujjwal96/mitmproxy,ddworken/mitmproxy,MatthewShao/mitmproxy,ddworken/mitmproxy,Kriechi/mitmproxy,vhaupert/mitmproxy,cortesi/mitmproxy,ujjwal96/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy,StevenVanAcker/mitmproxy,mhils/mitmproxy,vhaupert/mitmproxy,zlorb/mitmproxy,StevenVanAcker/mitmproxy,Kriechi/mitmproxy,vhaupert/mitmproxy,ddworken/mitmproxy,ujjwal96/mitmproxy,mitmproxy/mitmproxy,ujjwal96/mitmproxy,MatthewShao/mitmproxy,mhils/mitmproxy,cortesi/mitmproxy