fix python3 requirement.txt yaml version error

requirements.txt

 beautifulsoup4==4.5.3
 bs4==0.0.1
-PyYAML>5.1.1
+PyYAML==5.1.1
 six==1.10.0
 requests==2.18.4
 Pillow
@@ -12,4 +12,4 @@ keras>=2.2.4
 tensorflow
 matplotlib>=3.0.2
 numpy>=1.14.6
-scipy>=1.1.0
+scipy>=1.1.0
\ No newline at end of file