Unsupported bound type `MI` in `pulp.LpProblem.fromMPS`

Details for the issue
--------------------

#### What did you do?
I attempted to read an `.mps` file using `pulp.LpProblem.fromMPS`.

I discovered the issue when converting problems to `pulp` style `.json` files, and an `.mps` file containing an `MI` type bound caused `pulp` to error. `MI` bounds specify that the lower bound is `-inf` for a variable ([see note E here](https://lpsolve.sourceforge.net/5.5/mps-format.htm))

##### MPS File bounds section (simplified):

```
BOUNDS
...
MI bnd d_losvq_trnsf
UP bnd d_losvq_trnsf 400
...
ENDATA
```

#### What did you expect to see?
Successful file read.

#### What did you see instead?
`IndexError` raised.

In the function `readMPSSetBounds`, a series of checks are made against the bound type. Bound types that do not specify a bound value are caught and handled separately (currently `FR`, `BV`, and `PL` are caught). If the type is not caught, the bound value is read from the `lines` object, which is a list.

Since the `MI` bound type does not specify a bound value, and it is not caught by the if statements, an index error is raised since the `lines` list is an unexpected length.

### Proposed Solution

Adding an additional clause to `readMPSSetBounds` in `pulp.mps_lp`.

```python
def readMPSSetBounds(line, variable_dict):
bound = line[0]
var_name = line[2]

def set_one_bound(bound_type, value):
variable_dict[var_name][BOUNDS_EQUIV[bound_type]] = value

def set_both_bounds(value_low, value_up):
set_one_bound("LO", value_low)
set_one_bound("UP", value_up)

if bound == "FR":
set_both_bounds(None, None)
return
elif bound == "BV":
set_both_bounds(0, 1)
return
elif bound == "PL":
# bounds equal to defaults
return
elif bound == "MI":
# Lower bound -inf
set_one_bound("LO", None)
return

value = float(line[3])
if bound in ["LO", "UP"]:
set_one_bound(bound, value)
elif bound == "FX":
set_both_bounds(value, value)
return
```

I am uncertain if this causes side effects, as I have not tested the solution against the unit-tests myself. I have successfully used this as a "patch" to finish my conversion effort.

I couldn't find any documentation discussing the support of `MI` bound types, so my assumption is that they are supported. If I am incorrect, disregard this issue.

Useful extra information
-------------------------

#### What operating system are you using?

- [X] Windows: (11, Version 10.0.22631 Build 22631)

#### I'm using python version:

- [X] 3.11

#### I installed PuLP via:

- [X] pypi (python -m pip install pulp)
- [X] Other: conda (conda environment, pulp is installed via pip, no other packages)

#### Did you also

- [ ] Tried out the latest github version: https://github.com/coin-or/pulp
- [X] Searched for an existing similar issue: https://github.com/coin-or/pulp/issues?utf8=%E2%9C%93&q=is%3Aissue%20

Start in pulp.mps_lp at readMPSSetBounds and review the existing unit tests for MPS bounds handling. Reproduce the MI-bound example, verify that reading the file no longer raises IndexError, and run the unit tests to check for side effects.

Written by the indexing model from the issue text.