Bug report

Bug summary
I tried to use a previously-works-fine code to make a plot but failed to reproduce the same one -- after updating matplotlib to 2.2. The hollow circles in plot (defined by markerfacecolor='None') are now in black facecolor. After manually locating the error, I found that the keyword 'alpha' seems conflict to markerfacecolor = 'None' which never happened before. This conflict will only affect facecolor set as 'None'.

Code for reproduction

import matplotlib.pyplot as plt
# This works properly
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='r',linewidth=2,alpha=1)
plt.show()
# markerfacecolor='none' conflicts with alpha keyword
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='none',linewidth=2,alpha=1)
plt.show()
# markerfacecolor='none' conflicts with alpha keyword, and this happens on different alphas
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='none',linewidth=2,alpha=0.5)
plt.show()
# remove alpha keyword and markerfacecolor='none' works well
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='none',linewidth=2) # no alpha here
plt.show()

Actual outcome

![1](https://user-images.githubusercontent.com/13745875/39099965-ee512c2c-4650-11e8-9cc4-969ebe75ac2a.png)
![2](https://user-images.githubusercontent.com/13745875/39099966-ee604bb2-4650-11e8-9245-d846bbed9523.png)
![3](https://user-images.githubusercontent.com/13745875/39099968-ee6aa2f6-4650-11e8-8420-bde423460955.png)
![4](https://user-images.githubusercontent.com/13745875/39099969-ee73fbbc-4650-11e8-9358-72e00202e539.png)

Matplotlib version

• Operating system: Windows 10 x64
• Matplotlib version: 2.2
• Matplotlib backend (print(matplotlib.get_backend())): both terminal ipython (Qt5Agg) and jupyter notebook (module://ipykernel.pylab.backend_inline)
• Python version: 3.6.2
• Jupyter version (if applicable): 4.3.0
• Other libraries: N/A

matplotlib and python installed by conda

channel by default

Thanks a lot!