`show_doc` errors on Python 3.10 union sub-type hints

`show_doc` errors out when using Python 3.10 union sub-type hints. This occurs with typing.List `List[Tensor|int]` or list `list[Tensor|int]`

Minimal reproducible code and error:
```python
from __future__ import annotations
from nbdev.showdoc import *

def test(l:list[float|int]):
return l

show_doc(test)
```
Error:
```
Traceback (most recent call last):

Cell In [3], line 1
show_doc(test)

File ~/python3.9/site-packages/nbdev/showdoc.py:191 in show_doc
else:return renderer(sym or show_doc, name=name, title_level=title_level)

File ~/python3.9/site-packages/nbdev/showdoc.py:127 in __init__
try: self.sig = signature_ex(sym, eval_str=True)

File ~/python3.9/site-packages/fastcore/basics.py:350 in signature_ex
ann = type_hints(obj)

File ~/python3.9/site-packages/fastcore/basics.py:318 in type_hints
return {k:_eval_type(v,glb,loc) for k,v in ann.items()}

File ~/miniconda3/envs/fastai/lib/python3.9/site-packages/fastcore/basics.py:318 in
return {k:_eval_type(v,glb,loc) for k,v in ann.items()}

File ~/python3.9/site-packages/fastcore/basics.py:311 in _eval_type
res = eval_type(t, glb, loc)

File ~python3.9/site-packages/fastcore/basics.py:304 in eval_type
if '|' in t: return Union[eval_type(tuple(t.split('|')), glb, loc)]

File ~/python3.9/site-packages/fastcore/basics.py:306 in eval_type
if isinstance(t,(tuple,list)): return type(t)([eval_type(c, glb, loc) for c in t])

File ~/python3.9/site-packages/fastcore/basics.py:306 in
if isinstance(t,(tuple,list)): return type(t)([eval_type(c, glb, loc) for c in t])

File ~/python3.9/site-packages/fastcore/basics.py:305 in eval_type
return eval(t, glb, loc)

File :1
list[float
^
SyntaxError: unexpected EOF while parsing
```

However, using optional `list[Optional[float]]` or union `Union` `list[Union[float, int]]`works.

```python
from typing import Union
from nbdev.showdoc import *

def test(
l:list[Union[float, int]] # test comment
):
return l

show_doc(test)
```

Using [user-defined generic types](https://docs.python.org/3/library/typing.html#user-defined-generic-types) renders the table, but displays `list[T]` as the type, not `list[int, float]` (or something similar).

```python
from typing import TypeVar
from nbdev.showdoc import *

T = TypeVar('T', int, float)

def test(
l:list[T] # test comment
):
return l

show_doc(test)
```