Cannot run tests on HEAD due to `object.__init__() takes no parameters` on `BaseOffset.__init__(self, n, normalize)`
#### Code Sample, a copy-pastable example if possible

```shell
$ pytest pandas/tests/io/test_gbq.py
```

<details>

```shell
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/Users/swast/.pyenv/versions/3.6.4/envs/pandas-HEAD/lib/python3.6/site-packages/_pytest/config/__init__.py", line 406, in _importconftest
    mod = conftestpath.pyimport()
  File "/Users/swast/.pyenv/versions/3.6.4/envs/pandas-HEAD/lib/python3.6/site-packages/py/_path/local.py", line 668, in pyimport
    __import__(modname)
  File "/Users/swast/src/pandas/pandas/pandas/__init__.py", line 42, in <module>
    from pandas.core.api import *
  File "/Users/swast/src/pandas/pandas/pandas/core/api.py", line 10, in <module>
    from pandas.core.groupby.groupby import Grouper
  File "/Users/swast/src/pandas/pandas/pandas/core/groupby/__init__.py", line 2, in <module>
    from pandas.core.groupby.groupby import (
  File "/Users/swast/src/pandas/pandas/pandas/core/groupby/groupby.py", line 46, in <module>
    from pandas.core.index import (Index, MultiIndex,
  File "/Users/swast/src/pandas/pandas/pandas/core/index.py", line 2, in <module>
    from pandas.core.indexes.api import *
  File "/Users/swast/src/pandas/pandas/pandas/core/indexes/api.py", line 4, in <module>
    from pandas.core.indexes.base import (Index,
  File "/Users/swast/src/pandas/pandas/pandas/core/indexes/base.py", line 7, in <module>
    from pandas._libs import (lib, index as libindex, tslib as libts,
  File "pandas/_libs/index.pyx", line 27, in init pandas._libs.index
    from pandas._libs import algos, hashtable as _hash
  File "pandas/_libs/tslibs/period.pyx", line 59, in init pandas._libs.tslibs.period
  File "/Users/swast/src/pandas/pandas/pandas/tseries/offsets.py", line 2327, in <module>
    offset=BDay(), time_rule=None):
  File "/Users/swast/src/pandas/pandas/pandas/tseries/offsets.py", line 484, in __init__
    BaseOffset.__init__(self, n, normalize)
TypeError: object.__init__() takes no parameters
ERROR: could not load /Users/swast/src/pandas/pandas/pandas/conftest.py
```

</details>

```shell
$ pip freeze
```

<details>

```shell
alabaster==0.7.10
appnope==0.1.0
atomicwrites==1.1.5
attrs==17.4.0
Babel==2.5.3
backcall==0.1.0
cachetools==2.0.1
certifi==2018.4.16
chardet==3.0.4
colorlog==2.10.0
cycler==0.10.0
decorator==4.2.1
docutils==0.14
flake8==3.5.0
-e git+git@github.com:tswast/google-cloud-python.git@7f648ba34925f48a5e43cfc866413ce6cf1e0cc4#egg=google_api_core&subdirectory=api_core
google-auth==1.4.1
google-auth-oauthlib==0.2.0
google-cloud-bigquery==0.32.0
-e git+git@github.com:tswast/google-cloud-python.git@7f648ba34925f48a5e43cfc866413ce6cf1e0cc4#egg=google_cloud_core&subdirectory=core
google-resumable-media==0.3.1
googleapis-common-protos==1.5.3
idna==2.6
imagesize==1.0.0
ipython==6.3.1
ipython-genutils==0.2.0
jedi==0.11.1
Jinja2==2.10
kiwisolver==1.0.1
MarkupSafe==1.0
matplotlib==2.2.2
mccabe==0.6.1
mock==2.0.0
more-itertools==4.1.0
nox-automation==0.18.2
numpy==1.14.2
numpydoc==0.8.0
oauthlib==2.0.7
packaging==17.1
-e git+git@github.com:tswast/pandas.git@7829ad3290dc6894d24c1c853ffc4dabef50294a#egg=pandas
-e git+git@github.com:tswast/pandas-gbq.git@ade32a238d789a115788fe3e81e21aba34544a06#egg=pandas_gbq
parso==0.1.1
pbr==4.0.2
pexpect==4.4.0
pickleshare==0.7.4
pkginfo==1.4.2
pluggy==0.6.0
pockets==0.6.2
prompt-toolkit==1.0.15
protobuf==3.5.2.post1
ptyprocess==0.5.2
py==1.5.3
pyasn1==0.4.2
pyasn1-modules==0.2.1
pycodestyle==2.3.1
pyflakes==1.6.0
Pygments==2.2.0
pyparsing==2.2.0
pytest==3.6.2
python-dateutil==2.7.2
pytz==2018.4
requests==2.18.4
requests-oauthlib==0.8.0
requests-toolbelt==0.8.0
rsa==3.4.2
simplegeneric==0.8.1
six==1.11.0
snowballstemmer==1.2.1
Sphinx==1.7.4
sphinx-rtd-theme==0.3.0
sphinxcontrib-websupport==1.0.1
tqdm==4.23.4
traitlets==4.3.2
twine==1.11.0
urllib3==1.22
virtualenv==15.2.0
wcwidth==0.1.7
```

</details>

#### Problem description

I cannot run the tests due to this exception after updating to the latest commit. The stacktrace points to a line which was recently modified in https://github.com/pandas-dev/pandas/pull/21341. I'll try rebasing on an older commit to see if that fixes it.
