- replace "images" with "media"
- check not only .caption but also .caption_entities
and .parse_mode of individual media items

we should try to copy as little as possible:
if none of the `group_*` parameters was passed,
we don't need copying.

Otherwise, we only need to copy (the list and)
the first media item rather than all of them

rename to match arguments of functions that handle
individual media:

- group_caption -> caption
- group_caption_parse_mode -> parse_mode
- group_caption_entities -> caption_entities

make sure the original media group is intact

Check if the user explicitly set parse_mode
for the InputMedia* object.
Even if it is set to None, ValueError must be
raised.

add fixture for a media group without individual
captions rather than taking `media_group` fixtures
and setting attributes manually

this fixture can be used later when checking
defaults

* make `parse_mode` default to DEFAULT_NONE
in `Bot` and shortcut methods in classes
* exclude `parse_mode` from check in conftest.py
* add tests for handling of default values

to allow multiple calls with same media group,
I need to copy not only the first item
(that gets the caption) but all items because
otherwise bot with Defaults will assign its
default parse mode to every item,
which will cause ValueError upon subsequent calls

unless there are other things to be fixed,
this should be the commit that closes #3185

`_bot.py` had two imports (import copy and from copy import copy)

leave first import and replace `copy()` with `copy.copy()`
in code

Closes #3185

Refactoring becomes possible since #3305 was fixed.

Refactoring is based on ideas in commit
c38801f