Macでpython３インストールができません

Xcodeをインストール後、homebrewをインストール。Python3をインストールしようとしたら以下のようなエラーが出ました。
原因は何でしょうか？よろしくお願いします。

==> Installation successful!

==> Homebrew has enabled anonymous aggregate formulae and cask analytics.
Read the analytics documentation (and how to opt-out) here:
https://docs.brew.sh/Analytics
No analytics data has been sent yet (or will be during this install run).

==> Homebrew is run entirely by unpaid volunteers. Please consider donating:
https://github.com/Homebrew/brew#donations

==> Next steps:

Run brew help to get started
Further documentation:
https://docs.brew.sh

==> Tapping homebrew/cask
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask'...
remote: Enumerating objects: 586407, done.
remote: Counting objects: 100% (45/45), done.
remote: Compressing objects: 100% (35/35), done.
error: RPC failed; curl 56 LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 54
error: 470 bytes of body are still expected
fetch-pack: unexpected disconnect while reading sideband packet
fatal: early EOF
fatal: index-pack failed
Error: Failure while executing; git clone https://github.com/Homebrew/homebrew-cask /usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask --origin=origin --template= exited with 128.