add .flake8 to manifest file

[kennyballou]

Without this file, a few tests fail when running the test suite against
the source archive of the project. The specific failures stem from the
missing flake8 configurations adjusting the rules for the tests. The
errors in the test suite are resolved when this file is added. This can
be reproduced by running the following commands:

(/tmp)% curl -fsL \
        https://files.pythonhosted.org/packages/03/22/0fa6fd83033faf8010e9cd40c6fcd50f5b729ec49f0b7eca65b3b0551ef5/flake8-pyi-18.3.1.tar.gz \
        -o flake8-pyi-18.3.1.tar.gz
(/tmp)% tar -zxf flake8-pyi-18.3.1.tar.gz
(flake8-pyi-18.3.1)% python setup.py test
...
test_comparisons (tests.test_pyi.PyiTestCase) ... ok
test_defaults (tests.test_pyi.PyiTestCase) ... FAIL
test_empty_init (tests.test_pyi.PyiTestCase) ... ok
test_function_def (tests.test_pyi.PyiTestCase) ... FAIL
test_patched_flake8_clean_del (tests.test_pyi.PyiTestCase) ... ok
test_patched_flake8_clean_forward_refs (tests.test_pyi.PyiTestCase) ... ok
test_sys_platform (tests.test_pyi.PyiTestCase) ... ok
test_sys_versioninfo (tests.test_pyi.PyiTestCase) ... ok
test_typevar (tests.test_pyi.PyiTestCase) ... ok
test_vanilla_flake8_not_clean_del (tests.test_pyi.PyiTestCase) ... ok
test_vanilla_flake8_not_clean_forward_refs (tests.test_pyi.PyiTestCase) ... ok

======================================================================
FAIL: test_defaults (tests.test_pyi.PyiTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/flake8-pyi-18.3.1/tests/test_pyi.py", line 141, in test_defaults
    self.checkFileOutput('defaults.pyi', stdout_lines=stdout_lines)
  File "/tmp/flake8-pyi-18.3.1/tests/test_pyi.py", line 46, in checkFileOutput
    self.assertMultiLineEqual(expected, actual)
AssertionError: 'defaults.pyi:3:17: Y011 Default values for typed argument[160 chars]..."' != 'defaults.pyi:3:1: E302 expected 2 blank lines, found 0\nd[555 chars]nd 0'
+ defaults.pyi:3:1: E302 expected 2 blank lines, found 0
  defaults.pyi:3:17: Y011 Default values for typed arguments must be "..."
+ defaults.pyi:5:1: E302 expected 2 blank lines, found 0
+ defaults.pyi:7:1: E302 expected 2 blank lines, found 0
  defaults.pyi:7:20: Y011 Default values for typed arguments must be "..."
+ defaults.pyi:9:1: E302 expected 2 blank lines, found 0
+ defaults.pyi:11:1: E302 expected 2 blank lines, found 0
+ defaults.pyi:13:1: E302 expected 2 blank lines, found 0
- defaults.pyi:13:20: Y011 Default values for typed arguments must be "..."+ defaults.pyi:13:20: Y011 Default values for typed arguments must be "..."
?                                                                          +
+ defaults.pyi:15:1: E302 expected 2 blank lines, found 0

======================================================================
FAIL: test_function_def (tests.test_pyi.PyiTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/flake8-pyi-18.3.1/tests/test_pyi.py", line 124, in test_function_def
    self.checkFileOutput('emptyfunctions.pyi', stdout_lines=stdout_lines)
  File "/tmp/flake8-pyi-18.3.1/tests/test_pyi.py", line 46, in checkFileOutput
    self.assertMultiLineEqual(expected, actual)
AssertionError: 'emptyfunctions.pyi:5:5: Y009 Empty body should contain "..."[146 chars]..."' != 'emptyfunctions.pyi:4:1: E302 expected 2 blank lines, found 1[522 chars]..."'
+ emptyfunctions.pyi:4:1: E302 expected 2 blank lines, found 1
  emptyfunctions.pyi:5:5: Y009 Empty body should contain "...", not "pass"
+ emptyfunctions.pyi:7:1: E302 expected 2 blank lines, found 1
+ emptyfunctions.pyi:10:1: E302 expected 2 blank lines, found 1
+ emptyfunctions.pyi:14:1: E302 expected 2 blank lines, found 1
+ emptyfunctions.pyi:18:1: E302 expected 2 blank lines, found 1
  emptyfunctions.pyi:19:5: Y010 Function body must contain only "..."
+ emptyfunctions.pyi:21:1: E302 expected 2 blank lines, found 1
  emptyfunctions.pyi:23:5: Y010 Function body must contain only "..."

----------------------------------------------------------------------
...

Add the .flake8 configuration file to the MANIFEST.in file so the
flake8 configuration used for testing is also distributed in the source
archive.