[thci] bug fixes and clean up the code style (#3817)

This commit squashes the approved updates, mainly include
1) bug fix or enhancement
   - make pollperiod take effect
   - fix the issue in finding the address of the specific address
     (case-sensitive issue introduced in #3639)
   - make `__sendCommand` more robust for possible extra serial output

3) Update along with master code and other minor updates
   - the command used to find specific address (#3856)
   - the IP6Prefix string len (from 20 to 19)
   - correct typos

2) Code Style
   - use single quotes for code
   - update to use `print ()` and format `%`
   - format via with `black` tool locally
     `python3 -m black -l 79 -S tools/harness-thci -t py27`
   - add travis python check example for THCI with `flake8` tool

notes: require OT code with #3856 (get specific address) and #3862 (`joiner id`)

.travis.yml

@@ -83,6 +83,9 @@ matrix:
     - env: BUILD_TARGET="posix-mtd" VERBOSE=1 VIRTUAL_TIME=1
       os: linux
       compiler: gcc
+    - env: BUILD_TARGET="py-pretty-check"
+      os: linux
+      python: "2.7"
     - env: BUILD_TARGET="pretty-check"
       os: linux
       addons:

.travis/before_install.sh

@@ -63,6 +63,11 @@ cd /tmp || die
         clang-format --version || die
     }
 
+    [ $BUILD_TARGET != py-pretty-check ] || {
+        pip install --upgrade pip || die
+        python -m pip install flake8 || die
+    }
+
     [ $BUILD_TARGET != posix-app-pty ] || {
         sudo apt-get install socat expect || die
         JOBS=$(getconf _NPROCESSORS_ONLN)

.travis/script.sh

@@ -42,6 +42,10 @@ python --version || die
     make pretty-check || die
 }
 
+[ $BUILD_TARGET != py-pretty-check ] || {
+    flake8 --config=script/pystyle.cfg tools/harness-thci/ || die
+}
+
 [ $BUILD_TARGET != scan-build ] || {
     ./bootstrap || die
 

script/pystyle.cfg

@@ -0,0 +1,16 @@
+[flake8]
+# it's not a bug that we aren't using all of hacking, ignore:
+# E203: White space before ':' (see #python#black#315).
+# W503: Line break occurred before a binary operator (see #python#black#README.md).
+ignore = E203, W503
+
+# relax the length limitation for now.
+max-line-length = 119
+max-doc-length = 119
+
+# Print the source code generating the error/warning in question.
+show_source = True
+show-source = True
+
+# print the total number of errors.
+count = True