move MetadataUrl to pywps.app.Common to avoid dependencies on sphinx

Steps to Reproduce
```bash
conda create -n pywps4.2.9 python=3.7
conda activate pywps4.2.9
python setup.py develop
python
```

Then
```python
from pywps import tests
Python 3.7.9 | packaged by conda-forge | (default, Dec  9 2020, 21:08:20)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pywps import tests
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/david/src/pywps/pywps/tests.py", line 15, in <module>
    from pywps.ext_autodoc import MetadataUrl
  File "/home/david/src/pywps/pywps/ext_autodoc.py", line 2, in <module>
    from sphinx.ext.autodoc import ClassDocumenter, bool_option
ModuleNotFoundError: No module named 'sphinx'
```

pywps/app/Common.py

@@ -63,3 +63,21 @@ def __eq__(self, other):
             self.role == other.role,
             self.type == other.type,
         ])
+
+
+class MetadataUrl(Metadata):
+    """Metadata subclass to allow anonymous links generation in documentation.
+
+    Useful to avoid Sphinx "Duplicate explicit target name" warning.
+
+    See https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#anonymous-hyperlinks.
+
+    Meant to use in documentation only, not needed in the xml response, nor being serialized or
+    deserialized to/from json.  So that's why it is not directly in the base class.
+    """
+
+    def __init__(self, title, href=None, role=None, type_='simple',
+                 anonymous=False):
+        super().__init__(title, href=href, role=role, type_=type_)
+        self.anonymous = anonymous
+        "Whether to create anonymous link (boolean)."

pywps/ext_autodoc.py

@@ -5,22 +5,7 @@
 from sphinx.util import force_decode
 from docutils.parsers.rst import directives
 from pywps import Process
-from pywps.app.Common import Metadata
-
-
-class MetadataUrl(Metadata):
-    """Metadata subclass to allow anonymous links generation.
-
-    Useful to avoid Sphinx "Duplicate explicit target name" warning.
-
-    See https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#anonymous-hyperlinks.
-    """
-
-    def __init__(self, title, href=None, role=None, type_='simple',
-                 anonymous=False):
-        super().__init__(title, href=href, role=role, type_=type_)
-        self.anonymous = anonymous
-        "Whether to create anonymous link (boolean)."
+from pywps.app.Common import Metadata, MetadataUrl
 
 
 class ProcessDocumenter(ClassDocumenter):

pywps/tests.py

@@ -11,8 +11,7 @@
 from pywps import Process
 from pywps.inout import LiteralInput, LiteralOutput, ComplexInput, ComplexOutput, BoundingBoxInput, BoundingBoxOutput
 from pywps.inout import Format
-from pywps.app.Common import Metadata
-from pywps.ext_autodoc import MetadataUrl
+from pywps.app.Common import Metadata, MetadataUrl
 
 import re