Add python 3.7. Use general shebangs in scripts to test with python3;

Use 2.7_with_system_site_packages for python2 to run qt tests
as suggested in Travis doc instead of bypassing virtualenv by shebang.

.travis.yml

@@ -1,6 +1,9 @@
 language: python
+cache: pip
+dist: bionic
 python:
-  - "2.7"
+  - "2.7_with_system_site_packages"
+  - "3.7"
 addons:
   apt:
     packages:
@@ -11,7 +14,6 @@ addons:
       - xvfb
 install:
   - pip install -r requirements.txt
-  - ln -s src pybitmessage  # tests environment
   - python setup.py install
 script:
   - python checkdeps.py

checkdeps.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 """
 Check dependencies and give recommendations about how to satisfy them
 

src/bitmessagemain.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python2.7
+#!/usr/bin/env python
 """
 The PyBitmessage startup script
 """

src/tests/test_process.py

@@ -18,7 +18,7 @@ from common import cleanup
 def put_signal_file(path, filename):
     """Creates file, presence of which is a signal about some event."""
     with open(os.path.join(path, filename), 'wb') as outfile:
-        outfile.write(str(time.time()))
+        outfile.write(b'%i' % time.time())
 
 
 class TestProcessProto(unittest.TestCase):