Get test_singlesweep.py and test_multiwallet.py working on github CI

[mflaxman]

They work for me locally but don't work on GitHub CI (I temporarily disabled them). Not sure if it's an OS issue or something else.

$ pytest test_*.py -v
========================================== test session starts ===========================================
platform darwin -- Python 3.9.0, pytest-6.1.1, py-1.10.0, pluggy-0.13.1 -- /Users/mflaxman/.pyenv/versions/3.9.0/bin/python3
cachedir: .pytest_cache
rootdir: /Users/mflaxman/workspace/buidl-python
collected 10 items                                                                                       

test_multiwallet.py::MultiwalletTest::test_change_addr PASSED                                      [ 10%]
test_multiwallet.py::MultiwalletTest::test_debug PASSED                                            [ 20%]
test_multiwallet.py::MultiwalletTest::test_fail PASSED                                             [ 30%]
test_multiwallet.py::MultiwalletTest::test_receive_addr PASSED                                     [ 40%]
test_multiwallet.py::MultiwalletTest::test_seedpicker_basic PASSED                                 [ 50%]
test_multiwallet.py::MultiwalletTest::test_sign_tx PASSED                                          [ 60%]
test_singlesweep.py::SinglesweepTest::test_debug PASSED                                            [ 70%]
test_singlesweep.py::SinglesweepTest::test_fail PASSED                                             [ 80%]
test_singlesweep.py::SinglesweepTest::test_send_compressed PASSED                                  [ 90%]
test_singlesweep.py::SinglesweepTest::test_send_uncompressed PASSED                                [100%]

=========================================== 10 passed in 9.71s ===========================================

PR here for more details:
#62 (comment)