Add description for the module lxml

tests/README-TESTS.md

@@ -39,6 +39,7 @@ for Python 2).
 
 Then navigate to the `tests/` subdirectory and run:
 ``` shell
+python3 -m pip install -r requirements.txt
 python3 runner.py
 ```
 

tests/bash_tests/testcases.py

@@ -950,6 +950,13 @@ set Exif.Photo.DateTimeDigitized 2020:05:26 07:31:42
 
     def stdin_test(self):
         # Test driver for stdin
+        try:
+            import lxml
+        except ModuleNotFoundError:
+            print('ignored')
+            print('Missing module lxml, please install: `pip install lxml`')
+            return
+
         out     = BT.Output()
         a       = 'exiv2-bug1229.jpg'  # jpg with 2 APP1/xap segments
         b       = 'girl.jpg'

tests/bash_tests/utils.py

@@ -252,7 +252,6 @@ def md5sum(filename):
 def pretty_xml(text, encoding=None):
     """
     Add indent to the XML text
-    Required installation: pip install lxml
     """
     from lxml import etree
     encoding     = encoding or Config.encoding