Add a 'stubtest' flag to METADATA.toml (#6687)

Co-authored-by: Akuli <akuviljanen17@gmail.com>

stubs/aiofiles/METADATA.toml

@@ -1,2 +1,3 @@
 version = "0.7.*"
 requires = []
+stubtest = false  # easily fixable, some platform differences between local and CI

stubs/pyaudio/METADATA.toml

@@ -1,2 +1,3 @@
 version = "0.2.*"
 python2 = true
+stubtest = false  # install fails locally

stubs/pycurl/METADATA.toml

@@ -1 +1,2 @@
 version = "7.44.*"
+stubtest = false  # install failure, missing libcurl

stubs/six/METADATA.toml

@@ -1,2 +1,3 @@
 version = "1.16.*"
 python2 = true
+stubtest = false  # TODO: figure out why this is needed