Merge #14954: build: Require python 3.5

fa2797808e test: Remove python3.4 workaround in feature_dbcrash (MarcoFalke)
dddd1d05d3 .python-version: Specify full version 3.5.6 (MarcoFalke)
faa7cdf764 scripted-diff: Update copyright in ./test (MarcoFalke)
fa0e65b772 scripted-diff: test: Remove brackets after assert (MarcoFalke)
fab5a1e0f4 build: Require python 3.5 (MarcoFalke)
fa6bf21f5e scripted-diff: test: Use py3.5 bytes::hex() method (MarcoFalke)

Pull request description:

  Python 3.4 is EOL after March 2019, so switch to 3.5. See https://devguide.python.org/#status-of-python-branches

  This pull does the following in a bunch of commits:
  * scripted diff to use the `bytes::hex()` method in place of previous wrappers (`b2x`, `bytes_to_hex_str`, `hexlify`, ...)
  * Update the build system (gitian and travis) to remove python2.7 and replace it with python3.5
  * Another scripted-diff to remove brackets after `assert`. This is unrelated to the python3.5 switch, but a stylistic commit, so probably not worth to split up. The motivation behind it is to avoid asserting on data structures (such as tuples of length one), which never fails:
  ```py
  >>> assert(False,)   # with brackets
  >>> assert False,    # without brackets
  SyntaxError: invalid syntax
  >>> assert False     # proper assertion
  AssertionError
  ```
  * And then a final scripted diff to update the copyright headers in the `test` subfolder, since I touched most of the files anyway and it wouldn't make sense to split this commit out into a separate pull.

  For reference (contributed by luke-jr):

  Ubuntu LTS (bionic): 3.6.5
  Debian stable (stretch): 3.5.3
  RHEL 8 (expected before v0.19): 3.6.x
  Gentoo stable: 3.6.5
  Arch: 3.7.1

Tree-SHA512: 643c28cd2d5b9543ce4bf8ad2a8b282bc79b37dc5b25c9c8358e6ce201e2a67a546463e5f3430b16652eb2489d7c3ed4b0772cd2e2bf790fe68a5e3cc8a25029

contrib/devtools/README.md

@@ -167,7 +167,7 @@ still compatible with the minimum supported Linux distribution versions.
 
 Example usage after a gitian build:
 
-    find ../gitian-builder/build -type f -executable | xargs python contrib/devtools/symbol-check.py 
+    find ../gitian-builder/build -type f -executable | xargs python3 contrib/devtools/symbol-check.py
 
 If only supported symbols are used the return value will be 0 and the output will be empty.
 

contrib/devtools/symbol-check.py

@@ -9,7 +9,7 @@ still compatible with the minimum supported Linux distribution versions.
 
 Example usage:
 
-    find ../gitian-builder/build -type f -executable | xargs python contrib/devtools/symbol-check.py
+    find ../gitian-builder/build -type f -executable | xargs python3 contrib/devtools/symbol-check.py
 '''
 import subprocess
 import re