BLD Fix emcc warning on Python3.8 (#720)

.gitignore

@@ -11,6 +11,7 @@ geckodriver.log
 firefox/
 .vscode
 .idea
+.mypy_cache/
 
 build
 downloads

emsdk/patches/emcc-python38-warning.patch

@@ -0,0 +1,20 @@
+--- emsdk/emsdk/emscripten/tag-1.38.31/emcc.py	2019-04-24 16:21:45.000000000 +0200
++++ emsdk/emsdk/emscripten/tag-1.38.31/emcc.py	2019-04-24 16:21:45.000000000 +0200
+@@ -816,7 +816,7 @@
+           newargs[i] = newargs[i + 1] = ''
+           if key == 'WASM_BACKEND=1':
+             exit_with_error('do not set -s WASM_BACKEND, instead set EMCC_WASM_BACKEND=1 in the environment')
+-    newargs = [arg for arg in newargs if arg is not '']
++    newargs = [arg for arg in newargs if arg != '']
+ 
+     settings_key_changes = set()
+     for s in settings_changes:
+@@ -936,7 +936,7 @@
+ 
+     original_input_files = input_files[:]
+ 
+-    newargs = [a for a in newargs if a is not '']
++    newargs = [a for a in newargs if a != '']
+ 
+     # -c means do not link in gcc, and for us, the parallel is to not go all the way to JS, but stop at bitcode
+     has_dash_c = '-c' in newargs