[FIX] doc: mention pip3 instead of pip for python mod install suggestions

Do not suggest to use pip in the logger, the system administrator may use other
tools

Closes #19949

addons/calendar/models/calendar.py

@@ -927,7 +927,7 @@ class Meeting(models.Model):
             # FIXME: why isn't this in CalDAV?
             import vobject
         except ImportError:
-            _logger.warning("The `vobject` Python module is not installed, so iCal file generation is unavailable. Use 'pip install vobject' to install it")
+            _logger.warning("The `vobject` Python module is not installed, so iCal file generation is unavailable. Please install the `vobject` Python module")
             return result
 
         for meeting in self:

addons/phone_validation/tools/phone_validation.py

@@ -60,7 +60,7 @@ except ImportError:
         if not _phonenumbers_lib_warning:
             _logger.warning(
                 "The `phonenumbers` Python module is not installed, contact numbers will not be "
-                "verified. Try: pip install phonenumbers."
+                "verified. Please install the `phonenumbers` Python module."
             )
             _phonenumbers_lib_warning = True
         return number

doc/setup/install.rst

@@ -411,7 +411,7 @@ Source installation requires manually installing dependencies:
 
     .. code-block:: console
 
-        $ pip install -r requirements.txt
+        $ pip3 install -r requirements.txt
 
   - on OS X, you will need to install the Command Line Tools
     (``xcode-select --install``) then download and install a package manager
@@ -420,7 +420,7 @@ Source installation requires manually installing dependencies:
 
     .. code-block:: console
 
-        $ pip install -r requirements.txt
+        $ pip3 install -r requirements.txt
 
   - on Windows you need to install some of the dependencies manually, tweak the
     requirements.txt file, then run pip to install the remaning ones.