setup.py: extras=dev requires txtorcon on both py2 and py3

We used to avoid this on py3, but now that txtorcon (0.19.2) is compatible
with it, we can use it on both. This also means tests can rely on having
txtorcon available.

setup.py

@@ -1,24 +1,9 @@
-import sys
 from setuptools import setup
 
 import versioneer
 
 commands = versioneer.get_cmdclass()
 
-DEV_REQUIREMENTS = [
-    "mock",
-    "tox",
-    "pyflakes",
-]
-if sys.version_info[0] < 3:
-    # txtorcon is not yet compatible with py3, so we include "txtorcon" in
-    # DEV_REQUIREMENTS under py2 but not under py3. The test suite will skip
-    # the tor tests when txtorcon is not importable. This results in
-    # different wheels when built under py2 vs py3 (with different
-    # extras_require[dev] dependencies), but I think this is ok, since nobody
-    # should be installing with [dev] from a wheel.
-    DEV_REQUIREMENTS.append("txtorcon")
-
 setup(name="magic-wormhole",
       version=versioneer.get_version(),
       description="Securely transfer data between computers",
@@ -53,8 +38,8 @@
       ],
       extras_require={
           ':sys_platform=="win32"': ["pypiwin32"],
-          "tor": ["txtorcon"],
-          "dev": DEV_REQUIREMENTS, # includes txtorcon on py2, but not py3
+          "tor": ["txtorcon >= 0.19.2"],
+          "dev": ["mock", "tox", "pyflakes", "txtorcon >= 0.19.2"],
       },
       test_suite="wormhole.test",
       cmdclass=commands,