Add pyflyby fixer (using its tidy-imports script) (#4219)

* add pyflyby fixer

updates

* pyflyby: add docs

updates

* add tests to pyflyby fixer

doc/ale-python.txt

@@ -708,6 +708,52 @@ g:ale_python_pyflakes_auto_poetry           *g:ale_python_pyflakes_auto_poetry*
   if true. This is overridden by a manually-set executable.
 
 
+===============================================================================
+pyflyby                                                    *ale-python-pyflyby*
+
+g:ale_python_pyflyby_executable               *g:ale_python_pyflyby_executable*
+                                              *b:ale_python_pyflyby_executable*
+  Type: |String|
+  Default: `'tidy-imports'`
+
+  See |ale-integrations-local-executables|
+
+
+g:ale_python_pyflyby_options                     *g:ale_python_pyflyby_options*
+                                                 *b:ale_python_pyflyby_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to add command-line arguments to the pyflyby
+  tidy-imports invocation.
+
+
+g:ale_python_pyflyby_use_global               *g:ale_python_pyflyby_use_global*
+                                              *b:ale_python_pyflyby_use_global*
+  Type: |Number|
+  Default: `get(g:, 'ale_use_global_executables', 0)`
+
+  See |ale-integrations-local-executables|
+
+
+g:ale_python_pyflyby_auto_pipenv             *g:ale_python_pyflyby_auto_pipenv*
+                                             *b:ale_python_pyflyby_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
+g:ale_python_pyflyby_auto_poetry             *g:ale_python_pyflyby_auto_poetry*
+                                             *b:ale_python_pyflyby_auto_poetry*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a poetry, and set the executable to `poetry`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 pylama                                                      *ale-python-pylama*
 

doc/ale-supported-languages-and-tools.txt

@@ -466,6 +466,7 @@ Notes:
   * `pycodestyle`
   * `pydocstyle`
   * `pyflakes`
+  * `pyflyby`
   * `pylama`!!
   * `pylint`!!
   * `pylsp`

doc/ale.txt

@@ -3117,6 +3117,7 @@ documented in additional help files.
     pycodestyle...........................|ale-python-pycodestyle|
     pydocstyle............................|ale-python-pydocstyle|
     pyflakes..............................|ale-python-pyflakes|
+    pyflyby...............................|ale-python-pyflyby|
     pylama................................|ale-python-pylama|
     pylint................................|ale-python-pylint|
     pylsp.................................|ale-python-pylsp|