Allow for install_reqs line having whitespace at start.

[dbrnz]

This fixes:

$ buildozer android debug
# Check configuration tokens
# Ensure build layout
# Check configuration tokens
# Preparing build
# Check requirements for android
# Install platform
Traceback (most recent call last):
  File "/usr/local/bin/buildozer", line 11, in <module>
    load_entry_point('buildozer==0.36.dev0', 'console_scripts', 'buildozer')()
  File "/usr/local/lib/python3.6/dist-packages/buildozer/scripts/client.py", line 13, in main
    Buildozer().run_command(sys.argv[1:])
  File "/usr/local/lib/python3.6/dist-packages/buildozer/__init__.py", line 1059, in run_command
    self.target.run_commands(args)
  File "/usr/local/lib/python3.6/dist-packages/buildozer/target.py", line 92, in run_commands
    func(args)
  File "/usr/local/lib/python3.6/dist-packages/buildozer/target.py", line 102, in cmd_debug
    self.buildozer.prepare_for_build()
  File "/usr/local/lib/python3.6/dist-packages/buildozer/__init__.py", line 176, in prepare_for_build
    self.target.install_platform()
  File "/usr/local/lib/python3.6/dist-packages/buildozer/targets/android.py", line 467, in install_platform
    self._install_p4a()
  File "/usr/local/lib/python3.6/dist-packages/buildozer/targets/android.py", line 524, in _install_p4a
    deps = re.findall("^install_reqs = (\[[^\]]*\])", setup, re.DOTALL | re.MULTILINE)[0]
IndexError: list index out of range

[dbrnz]

In fact, this should fix issues #731 and #733.

[AndreMiras]

Looks good to me thanks, I'll give it a try and merge if all good for both p4a.stable and master. Seems to be, but we never know until we test :)

[AndreMiras]

Even though it doesn't install sh module on p4a stable branch. It's still a far better situation than before (well since I broke it 😂 ).