DyNet V1.1

Hi，
when I install Dynet1.1，I encountered a problem。
```
ERROR: Command errored out with exit status 1:
command: /home/xushiqiang/anaconda3/envs/svmrnn/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-31z7go6_/dynet_9cbb8ae7d6b44b6f8763a84cf638457c/setup.py'"'"'; __file__='"'"'/tmp/pip-install-31z7go6_/dynet_9cbb8ae7d6b44b6f8763a84cf638457c/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d /tmp/pip-wheel-vj590a3j
cwd: /tmp/pip-install-31z7go6_/dynet_9cbb8ae7d6b44b6f8763a84cf638457c/
Complete output (27 lines):
running bdist_wheel
running build
INFO:root:==============================
INFO:root:CMake path: /usr/bin/cmake
INFO:root:Make path: /usr/bin/make
INFO:root:Make flags: -j 4
INFO:root:Mercurial path: /usr/bin/hg
INFO:root:C compiler path: /usr/bin/gcc
INFO:root:CXX compiler path: /usr/bin/g++
INFO:root:---
INFO:root:Script directory: /tmp/pip-install-31z7go6_/dynet_9cbb8ae7d6b44b6f8763a84cf638457c
INFO:root:Build directory: /tmp/pip-install-31z7go6_/dynet_9cbb8ae7d6b44b6f8763a84cf638457c/build/py3.8-64bit
INFO:root:Library installation directory: /home/xushiqiang/anaconda3/envs/svmrnn/lib/python3.8/site-packages/../../..
INFO:root:Python executable: /home/xushiqiang/anaconda3/envs/svmrnn/bin/python
INFO:root:==============================
cmake version 3.16.3

CMake suite maintained and supported by Kitware (kitware.com/cmake).
g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

INFO:root:Creating build directory /tmp/pip-install-31z7go6_/dynet_9cbb8ae7d6b44b6f8763a84cf638457c/build/py3.8-64bit
INFO:root:Cloning Eigen...
abort: HTTP Error 404: Not Found
error: /usr/bin/hg clone https://bitbucket.org/eigen/eigen
----------------------------------------
ERROR: Failed building wheel for dynet
```
Could you give me some advices?When I used dynet2.1.2, I got `'dynet' has no attribute 'Saveable'`

Start with the setup.py installation path shown in the error and inspect the Eigen clone step, then compare it with the reported dynet2.1.2 Saveable attribute failure. Done means the installation path and the reported Python API behavior are separately understood and documented or reproduced with a focused test.

Written by the indexing model from the issue text.